Catalysis is critically important to energy production and to meeting the environmental quality mission of promoting the development and utilization of clean, efficient, and reliable energy resources. It is essential to understand the relationships between the atomic and nanoscale structure of metal nanoparticles and catalyst supports and the crucial role these play in promoting or altering catalytic pathways. The key focus of this talk lies in the controlled synthesis of metallic Au catalysts with unique metal-support interactions and nanostructured nonmetallic catalysts for heterogeneous catalysis. Critical issues and emerging science and technology in heterogeneous catalysis will be discussed in context of controlled synthesis.
Acknowledgement: This work was performed at the Oak Ridge National Laboratory and the University of Tennessee and supported by the U.S. Department of Energy&’s Office of Basic Energy Sciences under contract No. DE-AC05-00OR22725 with UT-Battelle, LLC.

Bimetallic nanoparticles possess catalytic properties different from those of constituent metals or simple mixtures, and often exhibit enhanced catalytic activity. Gold-based bimetallic materials integrate additional function to the catalysts because their strong interaction of light and open up the opportunities for plasmonic enhanced catalysis. When the optical “hot spots” overlap with the catalytically-active sites, one can expect tremendous enhancement of the catalytic activity under the optical resonance condition. In this work, we develop a kinetic-control seeded growth method to synthesize anisotropic gold-based materials, specifically, gold-copper nanorods for plasmonic enhanced catalysis. This approach uses gold nanoparticles as seeds, and subsequently reduces copper atoms on one side of the seeds, resulting in Janus nanoparticles with a gold-rich and a copper-rich portion. These Janus nanoparticles ultimately lead to the elongation of the nanoparticles into nanorods by retarding reduction kinetics of copper through galvanic replacement. The composition, crystal structure, and aspect ratio of the nanorods can be conveniently tuned by changing seed and precursor concentrations. A third metal such as platinum and palladium can be incorporated into the nanorods to form multi-metal anisotropic nanostructures. We compare the catalytic activity of these nanorods with different aspect ratio and crystal structures (alloy versus intermetallic) for the nitrophenol reduction and electro-oxidation of methanol. We demonstrate the plasmonic-enhanced catalytic activity of these nanorods upon irradiation of light. Mechanisms are further investigated by X-ray photoelectron spectroscopy and Raman spectroscopy.

Au based catalysts are widely studied due to their ability to catalyze selective reactions. Au catalysts, however, often exhibit low reactivity and are hindered by a low dissociation probability of small molecules. The activity can be enhanced by alloying Au with Pd, a more reactive but less selective catalyst. PdAu alloys are known to catalyze a wide range of hydrogenation and oxidation reactions; however, the size of Pd ensembles in Au required for small molecule activation is not well understood. Using scanning tunneling microscopy and temperature programmed desorption, we investigate the effect of the size of Pd ensembles in Au(111) on catalytic activation of H₂ and O₂. We show that isolated Pd atoms are capable of catalyzing the dissociative adsorption of H₂, a process which was previously thought to require contiguous Pd sites. The unique energy landscape of H₂ activation on the alloy results in an intermediate H₂ desorption temperature (170 K) compared to Pd(111) (310 K) and Au(111) (110 K). Compared to the well characterized PdCu single atom alloys known for allowing H spillover, Au based alloys do not enable H spillover from the active Pd sites to Au. H atoms can be transported between single Pd active sites which is suggestive of the transient existence of H atoms on Au. Conversely, single Pd atoms are not capable of O₂ dissociative adsorption. Small Pd clusters on Au enable O₂ adsorption at Pd-Au interface sites. Weakly bound H and O atoms are capable of enhancing reactions on the Au substrate and the PdAu system serves as an ideal model system to probe selective hydrogenation and oxidation reactions via single reactive sites.

Bulk gold is known to be catalytically inert whereas gold nanoclusters on metal oxide support can exhibit exceptional catalytic properties towards CO oxidation. We have performed a comprehensive study of catalytic activities of sub-nanometer gold clusters, hollow-cage clusters in particular, supported by TiO₂(110) surface using density functional theory calculations, Born-Oppenheimer molecular dynamics simulation, and micro-kinetics analysis. Our goal is to elucidate the importance in understanding the catalytic activity of subnanometer gold clusters with precisely controlled size and shape. We find that catalytic activities of the Au_n/TiO₂ systems increase with the size n up to 18, for which the hollow-cage Au₁₈ isomer exhibits highest activity for the CO oxidation, with a reaction rate ~100 times higher than that of Au₇/TiO₂ system. In stark contrast, the pyramidal isomer of Au₁₈ exhibits much lower activity comparable to the Au_3-4/TiO₂ systems. We will also show a Born-Oppenheimer molecular dynamics simulation evidence of the generation of oxygen vacancies at the hollow-cage Au₁₆ and TiO₂ (110) interface during CO oxidation. In this case, contrary to the conventional Langmuir-Hinshelwood (L-H) mechanism, the CO molecule adsorbed at the perimeter Au sites of Au₁₆ tends to attack a nearby lattice oxygen atom on the TiO₂ (110) surface rather than the neighboring co-adsorbed molecular O₂. So this simulation provides real-time demonstration of feasibility of the Mars-van Krevelen (M-vK) mechanism as evidenced by the generation of oxygen vacancies on the TiO₂ surface in the course of the CO oxidation. On the other hand, at the golden cage Au₁₈ and TiO₂ interface, the L-H mechanism is more favorable than the M-vK mechanism due to higher structural robustness of the Au₁₈ cage. In summary, the selection of either M-vK or L-H mechanism for the CO oxidation seems depending primarily on the structural flexibility of the hollow-cage Au clusters on the TiO₂ support.

Atomically well-defined gold nanoclusters protected by thiol ligands, referred to as Au_n(SR)_m, where n and m are specific numbers, exhibit quite unique catalytic activities compared to gold complexes and conventional gold nanoparticles. Thus far, gold nanoclusters have been demonstrated to have the capability of catalyzing a number of reactions in solution phase. As for the gas phase reactions such as carbon monoxide (CO) oxidation, supported Au_n(SR)_m nanoclusters on CeO₂ also show intriguing catalytic properties. One of the most attractive features for nanogold catalyzed CO oxidation is the size dependent catalytic activity. However, conventional gold nanocatalysts usually have a non-uniform particle size distribution, which would make it difficult to correlate the catalytic performance with the size and structure of the catalyst. Herein, we select several gold nanoclusters whose structures have been solved by X-ray crystallography, including Au₁₀(SR)₁₀, Au₂₅(SR)₁₈, Au₃₆(SR)₂₄ and Au₃₈(SR)₂₄, as a paradigm system to systemically study the size and structure dependent behavior in CO oxidation with CeO₂ supported Au_n(SR)_m nanoclusters as catalysts.

Converting waste CO₂ into fine chemicals and/or fuels is an exciting prospect for green-house gas mitigation. Current CO₂ conversion approaches include photo-, electro- and thermo-catalytic processes. Each approach has its benefits, but electrocatalytic CO₂ conversion is a leading candidate because it operates at ambient conditions with extremely high reaction rates, product selectivity and faradaic efficiency, and it requires small energy input (several volts). Carbon balance is a major consideration for any CO₂ conversion technology because the majority of the United States' electricity is derived from burning fossil fuels. This necessarily shifts most CO₂ conversion technologies into the carbon positive regime, and CO₂ produced from fossil-fuel derived electricity likely exceeds that consumed by the conversion process. This limitation is an important challenge that must addressed before CO₂ conversion technologies can be implemented on an industrial scale. Here, we describe a “carbon negative” (more CO₂ consumed than produced) electrochemical CO₂ conversion system based on atomically-precise Au₂₅ nanocatalysts. The catalytic system operates on inexpensive (ca. $10-20 USD), consumer-grade photovoltaic cells and solar-rechargeable batteries. We show stable CO₂ → CO conversion activity with turnover numbers (TON: mol CO₂/mol catalyst) surpassing 5x10⁶, rates exceeding 800 L CO₂ /gram catalytic metal/hour, and faradaic efficiencies and product selectivities around 90%. These performance metrics correspond to 1-2 kg CO₂ converted per gram of catalytic metal every hour. We can also tune product selectivity to favor CO or H₂ production by varying the catalyst loading and/or operating voltage. We show stable operation for 12 hours with photovoltaic cells to represent daylight operation, and 24 hours with a solar-rechargeable battery to mimic low-light or night-time hours. This study provides proof-of-principle example that renewable-powered electrocatalysis is a viable strategy for CO₂ conversion applications.

Thiolate-protected gold nanoclusters doped with foreign metal atoms have received considerable research interest in recent years. Doping of gold nanoclusters has drastic effects on their optical, electrochemical and catalytic properties, which can open up new avenues in their use in catalysis, sensing and biomedical applications. In this presentation, we report electrochemical properties of thiolate-protected Au₂₅ nanoclusters doped with Ag, Cu, Pt and Pd. Voltammetric investigations of the doped nanoclusters showed that redox potentials of gold nanoclusters are significantly altered by doping with foreign metal atoms. The electrocatalytic activities of the doped gold nanoclusters were examined by monitoring electrochemical reduction of azobenzene at doped gold nanocluster-modified electrodes. Dramatic mediated catalytic effect was observed with gold nanoclusters doped with Pt and Pd. This effect can be ascribed to the doping-induced tuning of the redox potentials and enhanced electrocatalytic activity of the doped gold nanoclusters.

Bimetallic nanoparticles (NPs), particularly Au/Pt and Au/Pd, have attracted extensive attention due to their wide-spread application in catalysis, optoelectronics and energy recuperation.[1] Here we have attempted the fabrication of Au/Pt and Au/Pd bimetallic NPs by an energy-efficient eco-friendly microwave methodology. The microwave-assisted reactions enable considerably large product yields over conventional colloidal methods due to (a) almost two-fold increased reaction kinetics, (b) localized superheating at reaction sites and rapid rise of initial temperature.[2] Au NPs (sizes 20 ± 3 nm) are fabricated in the first step followed by the reduction of [K₂PtCl₆] or [PdCl₂(NH3)₂] in tetraetylene glycol at 180 0C for 2 min. Controlling and understanding the atomic structure and elemental distributions of these NPs are crucial for their optimized performances. So, we also address the fundamental question of the most likely arrangement of Au and Pt or Pd atoms in these bimetallic NPs prepared under similar conditions by complementary characterizations using UV-Vis spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). The UV-Vis spectroscopy reveals the formation of an alloy shell. The extent of depression of the plasmon peak of Au and its blue-shift reveals substantial deposition of Pd atoms on an Au core and significant alloying in comparison to Au/Pt NPs. XRD reveals the gradual shift of the diffraction peak from the position of Au to the position of Pt or Pd with change in composition. XRD supports the formation of a thick alloy shell in these NPs. However, the TEM images reveal a very interesting result. With increase in Pt concentration, the size of the dispersed NPs decreases from 20 ± 3 nm to about 16 nm (± 1 nm) and there is evolution of a bimodal particle size distribution with small particles about 1-2 nm diameters. On the contrary, with increasing Pd concentration, the particle size of the dispersed particles increases to about 32 nm (± 1 nm). This discrepancy of particle size evolution for the two systems arises due to the differences in surface energies (Pt > Pd > Au atoms). Pt atoms tend to diffuse towards the core with the formation of Au nano-islands which eventually segregates leading to a reduction in particle size and bimodal distribution. At higher concentration of Pt, Pt and Au atoms tend to nucleate separately also contribution to bimodal distribution. While for Au/Pd NPs, we have an Au core with an alloyed shell having higher Pd concentration. This is further supported by experimental evidence by selective etching and dissolution of Au by potassium-iodide solution. Furthermore, the AuPd bimetallic NPs are found to posses better catalytic activities in the reduction of 4-nitrophenol to 4-aminophenol than AuPt and monometallic NPs.
References [1] R. Ferrando, et al Chem. Rev. 2008, 108, 845. [2] N. Dahal et al ACS Nano 2012, 6, 9433.

Surface-enhanced Raman spectroscopy (SERS) yields fingerprint vibrational information with ultra-high sensitivity up to single molecule level. When light of a resonant wavelength is incident upon gold nanostructures, it initiates surface plasmon resonance (SPR) that gives narrow regions of enhanced electromagnetic field strength, reshaping the incident laser dramatically. For widely extending the practical application of SERS, we adopted a “borrowing” strategy and rationally designed several kinds of gold nanostructures with controllable ultrathin shells, such as Au@SiO₂ nanoparticles, which is named “shell-isolated nanoparticle-enhanced Raman spectroscopy” or “SHINERS”. Here, the Au cores act to support SPR modes and boost electromagnetic field (EM) strength in localized regions at the interface. The ultrathin (2-5 nm) yet pinhole-free silica shells isolates the core from its chemical environment, but allow the EM field generated by the gold cores to probe that the surface and the environment. This method has been applied to a number of challenging systems, from metal single-crystal surfaces to semiconductors, from live cells to residue pesticides on fruits and vegetables.
These Au core-shell nanostructures with controllable ultrathin insulation shells are sensitive and uniquely suitable to provide a wealth of information about adsorption and reaction processes occurring at the surfaces, and to understand the correlations that exist between structure and reactivity for a wide range of interfacial phenomena. Recently we have extended the concept of shell-isolated gold nanoparticle-enhancement to several optical spectroscopies such as fluorescence, infrared absorption and sum frequency generation, showing great potential of plamon-enhanced spectroscopy of gold nanostructures with ultrathin insulation shells, which will be discussed in details.
References:
1. Jian-Feng Li et al., Nature, 2010, 464, 392-395.
2. Jian-Feng Li et al., Nature Protoc., 2013, 8, 52-65.
3. Zhong-Qun Tian, Bin Ren, Jian-Feng Li, Zhi-Lin Yang, Chem. Commun., 2007, 3514-3534.
4. Jian-Feng Li, Zhong-Qun Tian, in Frontiers of Surface-Enhanced Raman Scattering: Single Nanoparticles and Single Cells, eds. Y. Ozaki, K. Kneipp, R. Aroca, WILEY; Chichester, 2014, pp163-192.

Atomically precise thiolate-protected noble metal molecular nanoparticles are a promising class of model nanomaterials for catalysis, optoelectronics, and the bottom-up assembly of true molecular crystals. However, these applications have not fully materialized due to a lack of ligand exchange strategies that add functionality, but preserve the properties of these remarkable particles. In this presentation, we describe a method for the rapid (<30 s) and complete thiolate-for-thiolate exchange of the highly-sought-after silver molecular nanoparticle [Ag₄₄(SR)₃₀]^-4.We discuss the mechanism behind this rapid exchange that allowed us to preserve the precise nature of the particles, and simultaneously replace the native ligands with ligands containing a variety of functional groups. We also show how we were able to process the particles into smooth thin films, paving the way for their integration into solution-processed devices.

Recently, observation of switchable polarization-induced ferroelectric photovoltaic effect (FEPV) in BiFeO₃ (BFO) has attracted great interest. In order to improve its FEPV properties, co-substituted [Bi_0.9La_0.1][Fi_0.97Ta_0.03]O₃ (BLFTO) films were fabricated on Pt/TiO₂/SiO₂/Si substrates by pulsed laser deposition (PLD). The phase formation of the films was confirmed by X-ray diffraction, Raman spectroscopy, XPS studies. The FEPV properties in La and Ta co-doped BiFeO₃ (BFO) thin films were evaluated under illumination using sandwich configuration with ZnO:Al transparent conducting oxide (TCO) as top electrode. The TCO electrode was deposited by PLD using a shadow mask of 200 µm diameter. The band gap of the films was determined as 2.66 eV from spectrophotometric measurements, which is very close to that of pure BFO (2.7 eV). The ferroelectric nature of the films was characterized by P-E loop measurements, which indicated that leakage was still the prominent factor for not getting good polarization hysteresis. The piezo-force microscopy (PFM) studies however confirmed the ferroelectric nature of films. The open circuit voltage and short circuit current density were observed to be ~0.25 V and ~1.4 mA/cm², respectivly. The photovoltaic effect of the films was studied as a function of distance between the electrodes in the planar configuration. The photovoltaic effect decreased with increase in separation between electrodes. We will present aforementioned properties and will discuss the intended photovoltaic application of this non linear dielectric material in details.

Few atom fluorescent noble metal nanoclusters (<2nm in diameter) hold the promise for a variety of groundbreaking applications across many disciplines of science including optoelectronic devices, biocompatible probes for cellular imaging, and catalysis. The size-dependent optical and electronic properties of these nanoclusters make them attractive materials to study. Here we report the synthesis and characterization of DNA-templated fluorescent gold nanoclusters (AuNCs) with potential applications as biocompatible probes for cellular imaging and mediators of electron transfer for oxygen reduction reaction (ORR). Our initial characterization shows that these AuNCs are formed by few Au atoms ligated to one DNA molecule, and display a large Stokes shift of ~200 nm (lambda;_ex=430 nm, lambda;_em=630 nm) with a multicomponent lifetime in the range of ~0.4-4 mu;s. TEM images show that these clusters are sub-nm in size (<1 nm). The presence of both a Au(0) core, and a Au(I) surface is observed from XPS studies of these AuNCs, suggesting that these materials are truly cluster in nature. Changes in nitrogen speciation in the AuNC sample compared to the parent DNA hints that the nitrogen atoms of nucleobases ligate the NCs. Treatment of these AuNCs with a strong reductant results in a loss of the luminescence properties suggesting that the primary source of luminescence is the surface Au(I) atoms. Applications of these AuNCs as targeted fluorescent probes for cellular imaging and as electrocatalysts for the ORR are currently being investigated.

Nanoporous gold (np-Au) thin films, often produced by selective removal of silver from a gold-silver alloy, offer high electrical conductivity, catalytic activity, tunable pore morphology, and compatibility with conventional micropatterning techniques. These are critical properties for a number of important applications such as biosensors, fuel cells, and biomedical device coatings. In such applications, thin film coatings typically are applied on substrates with micro- to nano-scale features. As the precise control of pore morphology is essential for related applications, it is necessary to systematically investigate the effect of underlying substrate topography on np-Au thin films. In this study, we demonstrate a novel device architecture combing silicon surface patterns and sputter-coated np-Au films. Groove-based silicon gratings with groove widths varying between 150 nm to 50 µm were fabricated by photolithographic patterning, followed by pattern transfer to the underlying Si substrate by fluorine-based dry etching process. The micromachined silicon devices had an average ridge depth of 1.2 µm for gratings with groove widths above 500 nm, as confirmed by stylus-based profilometry. A 600 nm-thick gold-silver alloy film, as the precursor to the np-Au, was co-sputtered on the textured silicon chip. Grain size and distribution analysis by scanning electron microscopy coupled with digital image processing revealed that average grain size increased with increasing grating width. Subsequently, np-Au films with average pore size of 50 nm were obtained by dealloying the sputtered gold-silver film in nitric acid at 55°C. Dealloying step did not have a significant effect on pore moprhology as a function of grating width. Rapid thermal annealing was subsequently used to modify the pore morphology, in which heat facilitates surface diffusion of gold atoms leading to ligament and pore coarsening, which are hypothesized to be prone to mechanical constraints. Thermal treatment resulted in an increase in pore size for grating widths greater than 1 µm and preferential expansion of voids over pores in sub-micron gratings. In addition, the number of voids per unit area increased as the grating width decreased. In this study, we discuss the effect of surface topography on grain and pore morphology evolution as a function of thermo-mechanical constraints. The results include geometric, morphological, and elemental analysis of nanoporous gold films on the silicon substrate and the effect of thermal annealing at temperatures ranging from 200°C to 400°C. We expect these studies to provide insight into the nanostructured thin metal film behavior on textured substrates and facilitate cell-material interaction studies across nano- to micro-scale feature sizes.

Oxygen reduction reaction (ORR) based on Pt metal catalysts is most generally used in fuel cells. However, although their high activity, Pt metal based ORR has fundamental limitation due to its high cost and limited abundance. For these reasons, non-Pt catalysts such as other noble metallic (eg., Au, Ag), bimetallic (eg., Au-Co, Pd-Co), non metallic (eg., carbon nanotube) catalysts based ORR are also widely studied. However the catalytic activity and durability of the non-Pt catalysts remain unsatisfactory and thus the development of Pt-free catalytic ORR is considered as promising research area. In this poster, we compared the ORR activities for a variety of sized gold nanoparticles protected with hexanethiol (C₆S) ligands such as Au₂₅(C₆S)₁₈, Au₃₈(C₆S)₂₄, Au₆₇(C₆S)₃₅, Au₁₀₂(C₆S)₄₄, Au₁₄₄(C₆S)₆₀, and Au₃₃₃(C₆S)₇₉ by cyclic voltammetry (CV) and rotating disk electrode (RDE) techniques in basic media (0.1 M KOH). These well defined Au nanoparticels showed similarities and differences for their onset potentials, reduction currents, and number of electrons involved in the ORR. These size effects offer possibility to design new Pt-free ORR catalysts. Also we examined the catalytic activity of the Au nanoparticles immobilized in a sol-gel matrix that exhibited interesting activity for the oxygen electroreduction.

Graphene and its derivatives have been investigated extensively during the last decade because of their excellent thermal, mechanical and electric properties. Graphene also possesses very unique optical properties due to the linear dispersion relation and Dirac fermions which act as massless particles. Thus unique graphene plasmons can interact effectively with the incident photons. However, even though 2.3% of light absorption of single layer graphene is relatively high, it is required to enhance the light-matter interaction for the actual applications.¹ In this regard, the use of graphene in conventional plasmonic devices has been suggested.
SPR-based sensors are versatile research tools for molecular biology. It is highly sensitive to the refractive index (RI) of the medium and binding events (analyte-receptor) cause the change in RI resulting in the SPR angle shift enabling label-free sensing. Many efforts were carried out to overcome the limit such as difficulty in detection of small analytes and insufficient sensitivity (nanomolar detection) compared to ELISA. Incorporation of plasmonic nanomaterials is one of the most exploited strategies in label-free format.² Plasmonic nanomaterial-based SPR coupling induces the sensitivity by amplifying electromagnetic field.
In this work, graphene oxide (GO) and reduced graphene oxide (rGO) were introduced in SPR sensor chip in order to examine the coupling effect between graphene plasmons and propagating surface plasmons in Au film. Systematic and comparative studies on the deposition number and reduction of GO were conducted for the first time. Kretschmann configuration-based SPR sensor chip is composed of high refractive index glass prism and 48~50 nm of gold thin film. GO and rGO films were deposited on Au films by layer-by-layer self-assembly method and their RI sensitivity was compared in SPR-based sensors. Previous works revealed that graphene and its derivatives can enhance the sensitivity by increasing the attachment of biomolecules due to large surface area and by increasing electromagnetic field. By carrying out the bulk RI sensing, electromagnetic contribution of GO and rGO can be deconvoluted. The GO and rGO-deposited Au films showed different aspects in RI sensitivity: GO-Au film showed maximum sensitivity when the number of deposition was three while the RI sensitivity of rGO samples decreased with increasing number of layers. The biosensing activity was also performed with mass sensitivity of 1.43 ng mm^-2. Deposition of GO and rGO didn't hinder "figure of merit" of sensor chip which is common in plasmonic nanostructure-deposited Au film while the sensitivity was enhanced compared to neat Au film.
1. Grigorenko AN, Polini M, Novoselov KS. Graphene plasmonics.Nat Photon 2012, 6:749-758.
2. Zeng S, Baillargeat D, Ho HP, Yong KT. Nanomaterials enhanced surface plasmon resonance for biological and chemical sensing applications.Chem Soc Rev 2014, 43:3426-3452.

Several studies have been carried out in the last years aiming to a better and comprehensive understanding of structure-properties relationships of noble metal nanoparticles. In order to improve the knowledge about these relations, this work reports the surface modification of SBA-15 with an imidazolium ionic liquid-based alkoxysilane and its subsequent application in the synthesis of supported monometallic and bimetallic nanoparticles. The anion-exchange ability of the functionalized material was explored to promote the adsorption of Au and Pd anionic complexes on the porous framework, subsequently converted into metallic nanoparticles through the chemical reduction with NaBH₄. The catalytic activity of the resulting materials were tested in the reduction of 4-nitrophenol to 4-aminophenol in aqueous solution. The structural characterization of the catalysts were carried out by SAXS and nitrogen adsorption-desorption isotherms, which confirmed the ordered porous structure of SBA-15, with average pore diameter of 8 nm and specific surface area (BET) equal to 800 m² g^-1. No obstruction or blockage of the porous structure was observed after the functionalization and the synthesis of the supported nanoparticles. Elemental analysis demonstrated that the functionalization degree was 0.65 mmol g^-1. ICP-OES was carried out to quantify the weight percent of the loaded metals in the samples, which varied from 0.3 to 1.7 wt %. XRD and XPS were employed to estimate the crystallites sizes and to evaluate the formation of homogeneous or segregated phases. TEM images enabled the observation of the dispersion degree of the metallic nanoparticles through the silica support and showed that the pores acted as a template that controls the size of the nanostructures as well as a stabilizer that avoid the particles growth. The catalytic tests showed that the catalysts composed by Au_0.2Pd_0.8 and by Aushy;_0.4Pd_0.6 were more active than the samples containing only one metal.

During the past decade there has been a significant progress in the synthesis and characterization of nanocomposites, which can have unique properties due to the effect of the size of nanoparticles dispersed in a inorganic materials, such as clays and a polymeric matrix. Within this context, conducting polymers (CP) and montmorillonite clays are quite interesting materials in obtaining synergistic effects due to their relatively high electrical conductivity, chemical stability and low cost. For example, CP have been extensively employing to improve selectivity of modified electrodes in electrochemical sensor technology and act as an immobilizing matrix reducing interfering molecules. On the other hand, the use of gold nanoparticles (AuNPs) can increase the catalytic activity of sensors having modified electrodes, due to greater diffusion of electrons in the process. Here, nanocomposites formed from polyaniline (PAni), montmorillonite clay (MMTO) and gold nanoparticles (AuNPs) were obtained by the Langmuir-Blodgett (LB) technique, being characterized by UV-vis spectroscopy at each adsorbed bilayer. Results showed a good adsorption in the multilayer nanostructure formed, enabling the identification of PAni characteristic bands at ~ 300 nm (π-π^* transitions) and at approximately 800 nm, attributed to the formation of a polaronic band corresponding to the doped state of PAni (electron delocalization due to the conjugation formed in the polymeric backbone). The Raman and FTIR spectroscopy confirmed the presence of both materials and the good influence of AuNPs in PAni properties. The DRX results showed indicated the influence of AuNPs for intercalation of PAni in interlamelar space of clay sheets. The other hand, it was possible to verify that the conductivity of PAni was not affect by the presence of AuNPs. It was possible to identify the oxidation peaks of PAni at 0.2 to 0.5 V from electrochemical measurements acquired for the MMTO/AuNPs/PAni LB films soaked in an HCl 0.1 mol.L^-1 electrolyte solution, assigned respectively to the doped and undoped states of PAni by chloride ion. The sensor activity was evaluated by electrochemical measurements, being promising for application in environmental control for the detection of heavy metals, such as copper, lead and cadmium. These metals could be identified by standard potentials in approximately -0.04V for copper, -0.4V for lead and -0.7V for cadmium vs saturated calomel reference electrode (SCE).

In this work, we transformed Ag nanocubes into Ag-Au hollow nanocubes with a continuous shell of Ag-Au alloy on the surface and some remaining pure Ag in the interior. Upon removal of the pure Ag inside Ag-Au hollow nanocubes with aqueous H₂O₂, we obtained Ag-Au nanoboxes. We then systematically evaluated the SERS properties of the hollow nanocubes and nanoboxes by benchmarking against the Ag nanocubes. In one study, we collected the SERS spectra of 1,4-benzenedithiol (1,4-BDT) adsorbed on the surfaces of the nanoparticles when the samples were prepared using 1,4-BDT solutions with different concentrations. Our results showed that both the hollow nanocubes and nanoboxes exhibited much stronger SERS activity than the original Ag nanocubes. In particular, the remaining pure Ag inside the hollow nanocubes made a significant contribution to achieve SERS detection with sensitivity of 10^-11 M for 1,4-BDT. We further demonstrated their capability for SERS detection of melamine at 10^-8 M—a concentration much lower than the tolerance level of 1 ppm in infant formula. Additionally, we showed that the hollow nanocubes or nanoboxes with Ag-Au alloy shells on the surfaces were more stable than Ag nanocubes in an oxidative environment such as a solution containing an oxidant and/or halide ions. Taken together, these Ag-Au alloy nanostructures are good candidates for trace detection of biological and chemical analytes by SERS.

Nanoporous gold (np-Au) is an emerging material that has generated significant interest in the recent years due to its unique properties such a high electrical and thermal conductivity, high surface area-to-volume ratio, tunable pore morphology, and compatibility with microfabrication. These features made np-Au a popular material for fuel cells, biosensors, neural electrode coatings, and as a model system for studying nano-scale mechanics. Np-Au is produced by selective dissolution of silver from a gold/silver alloy, by a process known as dealloying, as gold atoms rearrange to create an open-pore morphology. Thermal treatment of the resulting np-Au structure increases surface diffusion of gold atoms and leads to coarser morphologies as a function of temperature and treatment duration. The extent of coarsening plays an essential role in the overall device operation and therefore precise control of morphology is necessary for attaining optimal device performance. Traditionally, thermal treatment in furnaces is used to obtain coarser morphologies, but this approach lacks the ability to create different morphologies on a single substrate. The ability to display multiple pore morphologies on a single substrate would greatly facilitate the identification of optimal morphologies that may enhance catalytic activity or surface-enhanced Raman signal from the metallic surface. Here, we present a novel technique based on electro-annealing to address this challenge and demonstrate several distinct morphologies on a single substrate. This technique is based on the principle that electrical current flowing in thin film traces lead to localized changes in current density as a function of trace geometry. At regions with smaller cross-section, increased current density leads to localized Joule heating. Electrical conductivity of np-Au thin films combined with rationally-designed trace geometries, allows for creating multiple and distinct pore morphologies via a short (less than 2 minutes), single treatment step performed at much lower (<150 #730;C) temperatures than those required for traditional thermal annealing (>250 #730;C). It is hypothesized that the reduced process temperature is achieved by electrically-assisted phenomena that contribute to thermally-assisted surface diffusion of gold atoms responsible for coarsening. Annealing at lower temperatures opens the door to the use of substrates with lower melting temperatures, such as many plastics. Overall, this approach can be easily scaled up to display multiple pore morphologies on a chip and should facilitate the production of high-throughput screening platforms on various substrates for studying structure-property relationships and identifying optimal morphologies for desired applications.

The gold/yttria-stabilized zirconia (Auminus;YSZ) nanocomposite system is a particularly interesting candidate for multifunctional applications. First, oxide-supported gold nanoparticles are very promising to dramatically lower the inset temperature of several catalytic reactions to room temperature or close to room temperature.¹ This approach currently stimulates many studies in the field of heterogeneous catalysis. Second, gold nanoparticles on different supports and gold/oxide nanocomposite films exhibit localized surface plasmon resonance (LSPR) in the visible range, making it useful for applications as a chemical sensing layer² and in the emerging field of plasmonics. Effectively, it enables combining the specific properties of YSZ, such as oxide ion conductivity, to the LSPR and catalytic activity provided by the gold nanoparticles. Third, as long as the gold loading is sufficient, annealing such films in air induces the relocation at the surface of some of the gold atoms to form a soft layer with adaptive tribological properties.³
This communication presents reactive magnetron co-sputtering syntehsis of Au-YSZ thin films, their optical, electrical,⁴ catalytic⁵ and frictional properties⁶ in connection to their microstructure and local chemistry. It is shown that:
- Thermally resilient LSPR can be achieved
- The electrical conductivity can be tuned by controlling the gold content, thermal annealing or laser interferrence patterning
- The produced layers can act can electrochemically promote the oxidation of methanol
- Drastic reduction of the friction coefficient can be obtained in as-deposited films, compared to pure YSZ, and further lowered using Laser interferrence patterning
1 Gold Bull. 37 (2004) 3minus;11
2 J. Phys. Chem. C 115 (2011) 6283minus;6289
3 Surf. Coat. Technol. 146minus;147 (2001) 351minus;356
5 ACS Appl. Mater.nter. 6 (2014) 13707-13715
6 J. Catal. 317 (2014) 293-302
7 J. Mater. Res. 27 (2012) 879-885

Iron oxide nanoparticles (NPs) have become increasingly important for cutting-edge applications in biomedicine due to both their biocompatibility and attractive magnetic properties. Super-paramagnetic magnetite nanocrystals possess high magnetic susceptibility, low remanence, low coercivity, and high saturation magnetization, making them ideal candidates for various biomedical applications including magnetic resonance imaging (MRI), magnetically controlled drug delivery and magnetic hyperthermia [1]. Manipulation of these NPs for in-vivo diagnostic and/or therapy assays requires their coupling through covalent bounding to bioactive molecules (antibodies, enzymes, proteins, DNAhellip;). This could be greatly simplified by coating these probes by a gold nanoshell which exhibits both high biocompatibility and affinity for binding to amine/thiol terminal groups of biorganic molecules [2].
Here, we report different facile and highly reproductive synthetic processes enabling the coating of polyol-made about 10 nm sized iron oxide NPs by a more or less continuous gold shell. The main strategy consists of grafting at the surface of these NPs charged molecules, namely positively charged (3-aminopropyl)triethoxysilane (APTES) and negatively charged citric acid, using sol-gel chemistry and surface complexation, respectively. These surface charges attract oppositely charged preformed gold particles or dissolved gold cations. In the first case discrete gold nanodots may be attached at the surface of the magnetic NPs, while in the second case, a continuous nanoshell may be produced around them. In all the cases, the morphological, optical and magnetic properties of the resulting hetero-nanostructures were investigated and described in relation with the quantity of deposited gold.

Nanoporous Gold (NPG) is a novel three dimensional, bicontinous and interconnected nanostructured monolithic material which serves as promising platform to design tailored catalysts via controlled surface modification through oxide deposition such as TiO₂. In this presentation, we will show that the catalytic activity of our NPG films (~150 nm thick) for H₂ oxidation is enhanced by depositing TiO₂ on them. We fabricated NPG films of varying porosity by de-alloying Si from Au_xSi_1-x alloy of varying Au/Si compositions. Our NPG films were further modified by titania deposition on the NPG films via titanium isopropoxide (TTIP) impregnation. In order to engineer catalytically active perimeter interface at the TiO₂/NPG junction, concentration of TTIP precursor was varied (up to 1 wt%) in liquid phase deposition. We found enhancement of H₂ oxidation activity for inverse TiO₂/NPG catalysts while pure NPG exhibited low turnover frequency (TOF). TOF increased as the weight fraction of TTIP was increased to 0.5 wt%. It however decreased for higher than 0.5 wt% TTIP, suggesting reduction of perimeter interface area between TiO₂ and NPG body, which serves as active sites.

The growing epidemic of antibiotic-resistant infections, combined with a decrease in the research and production of new antibiotics, continues to plague global healthcare systems.¹ There is a pressing need to develop alternative treatments for common drug-resistant bacteria for when standard antibiotic therapy fails. One such treatment is photodynamic therapy (PDT), which uses photosensitizers to kill bacteria or somatic cells through the production of cytotoxic singlet oxygen.² Previous research has shown that combining photosensitizers with nanomaterials,^{3, 4} including gold nanoparticles,⁵ greatly enhances their cytotoxic efficacy; however the mechanisms at work are poorly understood.
In this work a range of dyes, including some known photosensitizers, were introduced to a solution of gold nanoparticles synthesised via the citrate-reduction method. It was found that the nanoparticles interacted strongly with cationic dyes and in some cases enhanced absorption was observed during UV-visible spectroscopy (UV-Vis) observation of titration experiments. We also show that the addition of cationic dyes to gold nanoparticles triggers aggregation of the nanoparticles into large clusters up to several micrometres across. Nanoparticles were seen to fragment and agglomerate at high concentrations of cationic dye in the solution. None of these effects were observed when the nanoparticles were titrated with anionic dyes, nor when silver nanoparticles synthesised by the same method were titrated with either cationic or anionic dyes.
The three cationic dyes which showed enhanced absorption during UV-Vis - acridine orange, toluidine blue and crystal violet - are known photosensitizers. The phenomenon of enhanced absorption has been reported previously for toluidine blue and was shown to be accompanied by a marked increase in cytotoxicity.^{5, 6} It has not been previously reported for crystal violet and acridine orange, however, and singlet-oxygen generation measurements are planned to deduce the antimicrobial capabilities of these new conjugations.
1. B. Spellberg, R. Guidos, D. Gilbert, J. Bradley, H. W. Boucher, W. M. Scheld, J. G. Bartlett, J. Edwards and t. I. D. S. o. America, Clinical Infectious Diseases, 2008, 46, 155-164.
2. T. J. Dougherty, C. J. Gomer, B. W. Henderson, G. Jori, D. Kessel, M. Korbelik, J. Moan and Q. Peng, Journal of the National Cancer Institute, 1998, 90, 889-905.
3. A. C. Khazraji, S. Hotchandani, S. Das and P. V. Kamat, The Journal of Physical Chemistry B, 1999, 103, 4693-4700.
4. A. Khdair, B. Gerard, H. Handa, G. Mao, M. P. V. Shekhar and J. Panyam, Molecular Pharmaceutics, 2008, 5, 795-807.
5. S. T. Naima Narband, Ivan P. Parkin, Jesuacute;s Gil-Tomás, Derren Ready, Sean P. Nair and Michael Wilson, Current Nanoscience, 2008, 4, 409-414.
6. N. Narband, M. Uppal, C. W. Dunnill, G. Hyett, M. Wilson and I. P. Parkin, Physical Chemistry Chemical Physics, 2009, 11, 10513-10518.

This poster presents photocatalytic and electrocatalytic activities of noble metal nanoparticles (metal = Au, Ag, Pd, and Pt) anchored on the surface of ZnO nanoparticles. The ZnO-metal composites were prepared by anchoring glutathione-protected metal nanoparticles on ZnO surface and photocatalytic experiments were conducted with composite colloids under light (> 320 nm) in a deaerated 1:1 (v/v) EtOH-H₂O medium. The photocatalytic activity of bare ZnO for the reduction of azobenzene was found to be negligible, but it increased dramatically upon coating of metal nanoparticles. The enhancement was highest with Pd nanoparticles, followed by Pt, Au and Ag. Similar metal effect has been observed in the electrocatalytic reduction of azobenzene. Through product analysis of the photocatalytic reactions, it was found that azobenzene was reduced to hydrazobenzene via proton-coupled electron transfer reaction. The metal effect can be explained by the proton binding tendency of metals in the composites that controls the proton-coupled electron transfer reaction. Reduction of carbon dioxide which is considered as another proton-coupled electron transfer reaction is also discussed.

Compared with individual Au nanoparticle, Au nanoparticle clusters (AuNPCs) show efficient surface-enhanced Raman scattering (SERS) performance because they are rich in interparticle gaps, which can act as hot spots for strong electromagnetic field enhancement. Moreover, AuNPCs are very stable SERS platforms for biosensing applications. However, the lack of simple and reliable methods for preparing AuNPCs with well-defined structural parameters is a major obstacle for exploiting the unique plasmonic properties of AuNPCs toward practical applications. In practice, we applied AuNPCs on organic photovoltaic solar cell, and they show the high performance power conversion efficiency for the single active layer organic photovoltaics.¹
In the present work, we present a facile aqueous synthetic method for the preparation of AuNPCs by the controlled galvanic replacement of Ag nanoparticles with Au precursors. The final morphology of AuNPCs could readily be tuned by changing the structure of Ag nanoparticles. The prepared AuNPCs showed single-molecule level SERS detection and stability for various analytes both in water and under physiological conditions. We expect that the present study will provide a new direction for the development of an efficient plasmonic platform for biosensing and imaging and that it can also be applied to the fabrication of intriguing nanoarchitectures with desirable morphologies and functions.
Reference
1. H. I. Park, S. Lee, J. M. Lee, S. A. Nam, T. Jeon, S. W. Han, S. O. Kim, High Performance Organic Photovoltaics with Plasmonic-Coupled Metal Nanoparticle Clusters. ACS Nano in press.

Two dimensional (2D) atomic layered nanomaterials exhibit some of the most striking phenomena in modern materials research and hold promise for a wide range of applications including energy and device technology. More recently, graphene oxide-metal nanoparticle hybrid materials have been of increasing interest due to their potential applications in biological and environmental industries. Our research specifically targets the preparation of gold nanoparticle catalysts supported on 2D nanomaterials, including graphene oxide and bismuth selenide nanosheets, using the strategy of pre-formed nanoparticle self-assembly. This strategy is especially useful for fine tuning the size and monodispersity of gold nanoparticles that have a direct influence over the catalytic activity of the hybrid materials. Ultimately, the use of aforementioned 2D supports, having extremely high surface area to volume ratio, excellent electric conductivity, and unique characteristics of enhancing mass transport of reactants to catalysts, would maximize the activity of surface-assembled metal nanoparticle catalysts. As an initial step, the current research aims at understanding the interaction between ligand-capped gold nanoparticles and graphene oxide/bismuth selenide nanosheets. For this task, hydrophobic and hydrophilic ligand-capped gold nanoparticles with different core sizes are synthesized and mixed with 2D supports (graphene oxide or bismuth selenide nanosheets). By varying the functional groups on the ligands between alkyl, aromatic, amine and alcohol groups, we are able to induce different interactions such as Van der Waals, π-π stacking, dipole-dipole, and hydrogen bonding. The solvent and concentration effects on the interactions between 2D supports and nanoparticles are also tested. The adsorption of gold nanoparticles on to the surface of 2D supports was monitored using UV-vis spectroscopy and transmission electron microscopy (TEM). The results show that amine-functionalized hydrophobic ligand-capped gold nanoparticles produce well dispersed nanoparticles on the surface of graphene oxide without the formation of locally aggregated particle domains. The same nanoparticles are also found to have a strong interaction with bismuth selenide nanosheets forming gold nanoparticle-bismuth selenide hybrid nanostructures. Once the gold nanoparticles are anchored onto the 2D materials, the surface ligands are partially removed by annealing the nanoparticles at higher temperature, which enable the nanoparticles to act as supported catalysts. Future work will include understanding the catalytic behaviors of the produced hybrid nanomaterials for model organic reactions (e.g. oxidation of benzyl alcohol).

This presentation reports our observation of changes to the crystallinity or morphology during seed-mediated growth of Au nanocrystals. When single-crystal Au seeds with a spherical or rod-like shape were treated with a chemical species such as S₂O₃^2- ions, twin defects were developed during the growth process to generate multiply twinned nanostructures. X-ray photoelectron spectroscopy analysis indicated that the S₂O₃^2- ions were chemisorbed on the surfaces of the seeds during the treatment. The chemisorbed S₂O₃^2- ions somehow influenced the crystallization of Au atoms added onto the surface during a growth process, leading to the formation of twin defects. In contrast to the spherical and rod-like Au seeds, the single-crystal structure was retained to generate a concave morphology when single-crystal Au seeds with a cubic or octahedral shape were used for a similar treatment and then seed-mediated growth. The different outcomes are likely related to the difference in spatial distribution of S₂O₃^2- ions chemisorbed on the surface of a seed. This approach based on surface modification is potentially extendable to other noble metals for engineering the crystallinity and morphology of nanocrystals formed via seed-mediated growth.

Hollow gold nanospheres (HAuNS) and gold nanorods (AuNR) are promising contrast agents for photoacoustic tomography due to their strong and tunable absorbance in the optical window of biological tissues between 600 and 900 nm. HAuNS are obtained via reduction of tetrachloro auric acid by sodium borohydride, in the presence of citrate as capping ligands, on cobalt nanoparticles as sacrificial templates. AuNR are obtained following a seed-mediated growth process. For increased stability and possible biocompatibility that allow in vivo applications of HAuNS and AuNR, PEGylation via a ligand exchange with PEG (polyethylene glycol) ligands bearing thiols as anchor groups (PEG thiols) needs to be performed. We report the synthesis and the stabilization by mono-, bi- and tridentate PEG thiols of HAuNS with 35 nm diameter, having a shell thickness of 5.3 nm and an absorbance maximum at 852 nm, as well as the synthesis of AuNR with a length of 60 nm, having a width of 17 nm and a longitudinal absorbance maximum at 761 nm. Therefore, novel bi- and tridentate PEG thiols were synthesized comprising methoxy end groups and having similar molecular weights of around 5000 Da. The stability of these PEGylated HAuNS and AuNR was explored in three different aspects: (1) stability towards competition reactions with the strong binding ligand dithiothreitol (DTT), (2) resistance towards oxidative Au dissolution with potassium cyanide, and (3) colloidal stability, tested by addition of salt. These studies revealed improved colloidal stability for all PEGylated HAuNS and AuNR compared to citrate or CTAB stabilized ones, respectively and an increasing stability with increasing number of thiol anchor groups. Findings about the reduced cytotoxicity after PEGylation of all species as well as no significant difference in the cytotoxicity between the mono-, bi- and tridentate PEGylated particles complete this work.

Determining the total structures of nanoparticles constitutes a major goal in nanoscience research. Gold nanoparticles are particularly attractive due to their extraordinary stability and elegant optical properties. A prerequisite to total structure determination is to obtain atomically precise nanoparticles, which had been a major challenge in the past research and thus hampered the pursuit of fundamental science of nanoparticles. We have recently developed successful methodologies for synthesizing atomically precise gold nanoparticles protected by thiolates (denoted as Au_n(SR)_m, with n ranging from a few dozens to hundreds). Such ultrasmall nanoparticles (ca. 1-3 nm) exhibit distinct quantum size effects and interesting electronic/optical properties, which are fundamentally different from the plasmon-dominated properties of their larger counterparts (e.g. 5-100 nm nanoparticles). New types of atom-packing structures have been discovered in ultrasmall Au_n(SR)_m nanoparticles through X-ray crystallographic analysis. These well-defined nanoparticles hold potential in catalysis as new model catalysts, and atomic level structure-reactivity correlation will ultimately offer fundamental understanding on nanocatalysis.

Recently there have been considerable interests in the chemistry of atomically precise gold clusters with defined nuclearity and geometrical structures because they display molecular-like features that are virtually different from conventional colloidal nanoparticles. Phosphine-ligated gold clusters have a long history of studies to offer a library of diverse geometrical and electronic structures. X-ray crystal structures of monophosphine-ligated clusters have revealed that they generally favor to take icosahedron-based sphere-like geometries, but we recently found that the use of multidentate ligand sometimes allows the generation of exceptional non-spherical structures with anisotropic shapes. Here we show geometrical structures and electronic properties of such examples, which have nuclearity of 6, 7, 8 and 11. Unlike conventional sphere-like clusters such as icosahedral Au13, these anisotropic clusters all exhibit characteristic absorption band(s) in the visible region. The optical properties are strictly dependent on not only the nuclearity but also the cluster geometry. For some clusters bearing extra gold atoms attached to a polyhedral core (core+exo type), TD-DFT studies revealed that the exo gold atoms play a critical role in the emergence of unique electronic structure and distinctive optical features. We also report the syntheses of organic-functionalized cluster derivatives and their stimuli responsive functions.
ref) Struct. Bond., 2014, 161: 49-86; Angew. Chem. Int. Ed., 2011, 50, 7442; Chem. Commun., 2012, 48, 7559; Inorg. Chem., 2013, 52, 6570; J. Am. Chem. Soc., 2013, 135, 16078; J. Am. Chem. Soc., 2014, 136, 12892.

We present two topics on development of new ligand-protected Au clusters by controlling the interfacial structures.
1. Protection of Gold Clusters by Bulky Thiolates
Bulky arenethiols, EindSH (Eind = 1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl) and DppSH (Dpp = 2,6-diphenylphenyl), were used as protecting ligands with an aim to suppress the formation of -SR-[Au(I)-SR-]_n oligomers on the Au core. Au₄₁(SEind)₁₂ and Au₂₅(SDpp)₁₁ were obtained by mixing of the hydrosol of small (diameter<2 nm) PVP-stabilized Au clusters and the toluene solution of the corresponding thiol and subsequent etching process. As expected, the ligand-to-Au ratios of Au₄₁(SEind)₁₂ and Au₂₅(SDpp)₁₁ are significantly smaller than those of Au:SR reported so far. Structural characterization of these isolated clusters suggests that the bulky thiolates are bonded directly to the surface of Au₄₁ with twisted pyramidal motif and Au₂₅ with vertex-shared bi-icosahedral motif.
2. Protection of Gold Clusters by Terminal Alkynes
Gold clusters protected by terminal alkynes, 1-octyne (OC-H), phenyl acetylene (PA-H) and 9-ethynyl-phenanthrene (EPT-H), were prepared by the ligand exchange of the small (diameter<2 nm) Au:PVP clusters. Vibrational spectroscopy revealed that the terminal hydrogen is lost during the ligand exchange. A series of precisely-defined gold clusters, Au₃₄(PA)₁₆, Au₅₄(PA)₂₆, Au₃₀(EPT)₁₃, Au₃₅(EPT)₁₈, and Au_41-43(EPT)_21-23, were synthesized and characterized in detail to obtain further insight into the interfacial structures. An upright configuration of the alkynes on Au clusters was suggested from the Au to alkyne ratios and photoluminescence from the excimer of the EPT ligands. EXAFS analysis implied that alkynyl carbon is bound to bridged or hollow sites on the Au cluster surface.

Gold nanoclusters (Au NCs), containing only tens or hundreds of gold atoms, exhibit extraordinary optoelectronic and structural properties much different than the bulk and even larger gold nanoparticles [1]. These ultrasmall gold nanoparticles are promising for next generation of sensor devices and catalysts. Due to the ultra-small size regime of Au NCs (~1-2 nm), it is often challenging to understand the origin of their unique properties which are highly sensitive to size and composition [2]. Although recent advancements in Au NC synthesis have produced an extensive library of total structures from single-crystal X-ray diffraction characterization, a deeper understanding of Au NC local structure and electronic properties remains to be unlocked by probing metal and ligand environments via X-ray spectroscopy. Presented herein are recent synchrotron-based X-ray spectroscopy studies on the bonding properties of thiolate-protected gold nanoclusters (Au_n(SR)_m, SR-thiolate ligand) with varied composition (Au₁₉(SR)₁₃, Au₂₄(SR)₂₀ and Au₂₅(SR)₁₈) [3], a selenolate-protected surface (Au₂₅(SeR)₁₈) [4] and face centered cubic (FCC)-ordered core structures (Au₂₈(SR)₂₀, Au₃₆(SR)₂₄ and Au₄₄(SR)₂₈) [5,6]. As seen for Au₁₉ and Au₂₄, even a small change to the core composition or relative Au/SR ratio (compared to the Au₂₅ model) drastically changes the structural and electronic properties [3]. For both FCC-ordered core NCs and the selenolate-protected Au₂₅ NC, an unexpected thermal contraction property was detected with temperature dependent X-ray absorption experiments [4-6]. FCC-ordered core structures contain tightly bonded Au₄ units, important for rationalizing the thermal contraction of the metal core and the highly localized valence electronic structure. Selenolate-protected gold nanoclusters were found to have weak aurophilic interactions (compared to its thiolate-protected analogue, Au₂₅(SR)₁₈) on the surface that become stronger with increasing temperature, which potentially induce a contraction of the metal core. Furthermore, XAS and supporting theoretical calculations on site-specific ligand and metal environments offer a new and exciting perspective on the bonding properties gold nanoclusters.
[1] Jadzinsky et al. Science 2007, 318, 430minus;433.
[2] Qian et al. Acc. Chem. Res. 2012, 45, 1470minus;1479.
[3] Chevrier et al. J. Phys. Chem. C 2012, 116, 25137-25142
[4] Chevrier et al. J. Phys. Chem. C 2014, 118, 21730-21737
[5] Chevrier et al. J. Phys. Chem. C 2014, submitted
[6] Chevrier et al. J. Phys. Chem. Lett. 2013, 4, 3186-3191

Nanoparticles with their solution processability and size-tunable optical properties offer a wide range of opportunities for designing inexpensively fabricated photovoltaics and photocatalysts. On the other hand, even nominally monodisperse nanoparticles have a large degree of heterogeneity in the number of atoms of their core and ligands of their shell, which makes their properties difficult to model and predict. There is, however, a special class of homogeneous nanoparticles, known as molecular metal nanoparticles or atomically discrete nanoclusters, whose complete structure (core and shell) is exactly determinable and feasible to model. These precise materials have the potential to revolutionize the understanding of nanoparticle photovoltaics, catalysis, and charge transport properties. Unfortunately a) the number of reported molecular metal nanoparticles still remains modest; b) there are very few examples of such particles that are efficient in being both visible sensitizers and electron donors; and c) the particle&’s are usually synthesized with capping ligands and solvents that are unsuitable for the end application. Those are still major bottlenecks that have so far limited the utility of this promising class of nanomaterials.
In this presentation we reveal the total crystal structure and photophysical properties of a novel family of thiolate-protected molecular silver-gold nanoparticle alloys. Time-resolved photoluminescence and transient absorption spectroscopies were used to investigate the materials. The particles possess remarkably long photoluminescence lifetime (>150 ns), and have been found to be efficient electron donors when paired with small organic molecule acceptors. We show that with the right choice of ligands these particles can be used as efficient sensitizers for semiconductor systems utilized in the photocatalytic production of hydrogen. We discuss the role played by the ligand&’s structure and binding affinity to the metal, in determining the particle&’s size and promoting interfacial electron transfer.
The developments highlighted in this presentation pave the way for exploring practical applications of molecular noble metal nanoparticles as sensitizing donors in solution-processed photovoltaics and photocatalysts.

Nanomaterials which exhibit both stability and functionality are currently considered to hold the most promise as components of nanotechnology devices. Thiolate (RS)-protected gold nanoclusters (Au_n(SR)_m) have attracted significant attention in this regard and, among these, the magic clusters are believed to be the best candidates since they are the most stable. Our group has attempted to establish methods of modifying magic Au_n(SR)_m clusters, with the goal of creating nanomaterials which are both robust and functional, by increasing the functionality of the clusters. As shown in many previous gas phase studies, doping small metal clusters with heteroatoms has a significant effect on their stability as well as their physical and chemical properties. It was therefore anticipated that the properties of magic Au_n(SR)_m clusters would also be significantly modified by doping with heteroatoms. In addition, protection of magic Au_n(SR)_m clusters by selenolate ligands and functionalized thiolates was considered to be useful approachs. Therefore, we have investigated the effects of heteroatom doping, protection by selenolate ligands and protection by functionalized thiolates on the stability and physical/chemical properties of these clusters. Our studies, therefore, have essentially been aimed at devising methods of tailoring magic Au_n(SR)_m clusters to produce compounds which are stable and exhibit useful properties. In this presentation, I report our studies towards this goal and the obtained results.

The desire to fabricate materials with well-defined, controllable physicochemical properties and structures on a nanoscale has generated considerable interest in bimetallic nanoclusters or nanoalloys. Interactions between the constituent atoms can modify the alloy&’s electronic structure and surface composition, leading to enhanced chemical, catalytic, and optical properties. In particular, the gold-based bimetallic system in very small (< ~2 nm) nanoclusters has received much attention, since the alloying or segregation (that is, bimetallic core configuration) can lead to significant perturbations on the electronic structure as well as its chemical/catalytic properties.
From a viewpoint of cluster chemistry, on the other hand, recent advances in the selective synthesis of atomically precise, thiolate-protected gold nanoclusters with molecular purity have motivated researchers to investigate their curious structures and physicochemical properties. In the nanosystems, it is now accepted that they are composed of a metal core surrounded by oligomeric -[S-Au]_n-S- motifs often called “staples” or “semi-rings”. Therefore, the heteroatom doping (or alloying) in gold nanoclusters allows to probe the atomic sensitivity of the nanocluster&’s physical and chemical properties, as well as imparting the nanocluster with new properties.
The main objective in this study is to gain insight into the electronic structures and chiroptical responses of bimetallic Au-Pd and Au-Ag nanoclusters protected by chiral thiolate ligand, glutathione (GSH), and compare them with monometallic counterparts. The effect of Pd or Ag doping on the chiroptical responses of optically active Au nanoclusters as well as the importance of the bimetallic core configurations are discussed. Briefly, we find that GS-protected Au-Pd or Au-Ag nanoclusters exhibit quite different Cotton effects from those of the monometallic nanoclusters in metal-based electronic transition regions. In the Au-Pd system, all bimetallic nanoclusters exhibit featureless absorption profiles, but their circular dichroism (CD) signals were rather structured, offering a greater advantage in detecting a foreign atom doping in the nanocluster systems. In the Au-Ag system, interestingly, the nanocluster compounds exhibit weaker CD responses than those of the corresponding Au counterparts. The CD decrease can be explained in terms of the increased geometrical isomers that are formed by statistical distribution of Ag heteroatoms in the nanocluster, since an increased number of possible configurations is expected to give the average in the CD response with positive and negative bands of different optical isomers.

Structure prediction is the greatest challenge in the computational studies of thiolate-protected gold nanoclusters. Usually a simple ligand such as a methyl group is used to replace complex ligands for efficiency in exploring the relative stability of different isomers. However, the isomer stability may depend on the ligand group, which is particularly important for the large protecting ligand wherein the vdWs interactions play a significant role. A case in point is the Au₂₄(SR)₂₀ nanocluster. Here we show that changing the R group of Au₂₄(SR)₂₀ from CH₂CH₂Ph to CH₂Ph-^tBu leads to different lowest-energy isomers. Thus the ligand plays a crucial role in dictating the isomer stablility. This insight prompts us to perform the structural search of Au₁₈(SR)₁₄ cluster with the consideraction of both the simplified -CH₃ group and the real ligand (cyclo-hexane) used in experiment.

Gold, contrary to common belief, is a critically important industrial metal and is used widely in the electronics, chemical, medical and engineering sectors. In 2013, industrial demand for gold stood at ~400 tonnes, which represents ~10% of total demand across the entire market.
This paper will give a general overview of gold&’s place in technical applications in 2015, and will then focus on the challenges and opportunities in the development of new commercial uses, both from a scientific and non-scientific perspective. Specific examples of commercialisation efforts across a range sectors will be discussed, and the challenges experienced in some cases, particularly as a consequence of the wider gold market in recent years, will be highlighted. Finally, a brief review of the patent literature will be presented.

This talk will emphasize recent theory and experiments which have probed the effect of arrays of gold particles in 1D, 2D and 3D on optical response, with emphasis on the use of these array structures as metamaterials and in lasing applications. The arrays in 1D and 2D can be made using standard lithography tools, but much of the talk will emphasize bottom-up assembly of arrays that is possible using DNA-functionalized nanoparticles and self-assembly of nanoparticle superlattices driven by DNA hybridization. We show that the array structures lead to hybrid optical modes in which localized surface plasmon resonances in the nanoparticles are coupled with photonic modes of the lattices. These hybrid modes are often much narrower than the isolated particle plasmons, leading to greater field enhancements that are of interest in SERS, and greater refractive index sensitivity. In addition, we show that these modes make it possible to generate a new class of sub-wavelength lasers in which excitons in laser dyes are coupled with the hybrid lattice modes to produce enhanced stimulated emission. And finally we show new metamaterials properties in some of the lattices.

Au nanorods (AuNRs) with roughly a width of 20 nm and a strongly tunable aspect ratio have attracted much attention due to their unique optical and electronic properties, which depend on their aspect ratio, shape, and size [1]. Like spherical Au nanoparticles, AuNRs have the ability to absorb light of different wavelength ranges due to surface plasmon resonances (SPR). Due to the anisotropic shape of AuNRs, the SPR splits into two separate bands known as the transverse (TSPR) and longitudinal plasmon bands (LSPR). The TSPR is in the visible wavelength between 500 nm and 550 nm, while the LSPR varies from visible (600 nm) to near IR (>1200 nm), depending on the AuNRs aspect ratio.
The aim of our research is to study the packing of these AuNRs inside emulsion droplet; already this resulted for spherical nano-particles after droplet evaporation in a variety of supraparticles with intriguing icosahedral structures [2]. Electron tomography is used for observing the 3D structures quantitatively on the single nano-particle level. Particles in the reconstructed structures were tracked by an algorithm which could find the position, orientation, and length of each AuNR [3]. A thin shell of SiO₂ was applied as coating on the AuNRs before emulsification. Such a layer has three main advantages. First, the total aspect ratio can be tuned by the SiO₂ shell thickness. Second, a thin shell of SiO₂ is useful to tune the LSPR of single AuNRs and the plasmonic coupling inside the supraparticles. Finally, a SiO₂ shell also acts as a tunable spacer, which is helpful for the electron tomography and tracking, and for modifying the coupling of the plasmon resonances with other particles such as luminescent semiconductor particles that will be incorporated in binary supraparticles later.
The structures within the supraparticles highly depend on the aspect ratio of the AuNRs. When the aspect ratio is low (1.8), the supraparticles have a random packing of the nano-rods. However, supraclusters have a local smectic-like order when the aspect ratio of AuNRs is larger (e.g.: 4.7). With an increasing aspect ratio, one finds many more AuNRs organized side-by-side in 2D layers that themselves are stacked as well. This is consistent with computer simulations in which only a hard interaction between the rods and spherical confinement was taken into account.
Work is also in progress on the relationship between the structure and plasmonic properties of the supraparticles and on using these particles with sizes still in the colloidal domain in a second self-assembly step.
[1] C. J. Murphy, M. A. El-Sayed et al., The golden age: gold nanoparticles for biomedicine, Chem. Soc. Rev., 2012, 41, 2740.
[2] A. van Blaaderen, M. Dijkstra et al., Entropy-driven formation of large icosahedral colloidal clusters by spherical confinement, Nat. Mater., 2014, ASAP, DOI: 10.1038/NMAT4072.
[3] A. van Blaaderen et al., J. Phys.: Condens. Matter, 2014, accepted.

We show how vivid, highly polarized, and broadly tunable color pixels can be produced from periodic patterns of oriented gold aluminum nanorods. While the nanorod longitudinal plasmon resonance is largely responsible for pixel color, far-field diffractive coupling is employed to narrow the plasmon linewidth, enabling monochromatic coloration and significantly enhancing the far-field scattering intensity of the individual nanorod elements. The bright coloration can be observed with p-polarized white light excitation, consistent with the use of this approach in display devices. The resulting color pixels are constructed with a simple design, are compatible with scalable fabrication methods, and provide contrast ratios of at least 100:1. Compared to gold, aluminum is earth abundant, low in cost, compatible with complementary metal-oxide semiconductor manufacturing methods, and capable of supporting tunable plasmon resonance structures that span the entire visible spectrum.

Thermal transfer at the interface between solids and liquids is central to applications ranging from microelectronics cooling to targeted therapeutic heating. There has been a considerable amount of theoretical research on the thermal dynamics at solid-liquid interfaces, but experimental investigation has been limited by sensitivity constraints. Utilizing the well-known gold "breathing modes", or fundamental oscillations of thin-films and nanoparticles, we are able to analyze effects energy transfer from these solid structures to their environment. We use ultra-fast laser pulse excitation and environmental sensor configurations consisting of 20 nm gold thin films and nano-disks to excite fundamental oscillations in the gold structures and probe their changes in various environments. Using thin film sensors we have found that mechanical confinement of the gold surface by liquids can damp oscillations up to 60% and that despite being classically defined at highly wetting, the micro-mechanical properties of fluorinated liquids show limited mechanical interaction with solids on the nano-scale. To extend our analysis, we will use gold nano-disk sensors that exhibit lower frequency modes, enabling more sensitivity to other components of energy transfer at the interface. This combination of sensing configurations, all based on gold nano-structuring and ultra-fast pump-probe techniques allow us to deconstruct the mechanisms involved in transfer of thermo-mechanical energy from solids to liquids.

Colloidal nanostructures of plasmonic noble metals (e.g. gold and silver) exhibit localized surface plasmon resonance and display significant scattering and absorption in the visible and infrared spectrum. The plasmon excitation of these nanostructures is strongly dependent on their size, shape, and the chemical environment. For anisotropic nanostructures, the exact plasmon resonance mode is also very dependent on their orientation relative to the incident light. Such dependence provides enormous great opportunities for developing novel stimuli-responsive colorimetric devices. In this presentation I will introduce some of our recent works in designing stimuli responsive materials by taking advantage of the orientational dependent plasmonic property of Au nanorods and the interparticle near-field plasmon coupling effect of one-dimensional Au nanoparticle chains.

The direct optical monitoring of electron exchange on single plasmonic nanoparticles (NPs), involved in chemical reactions with gas molecules, is one of the main challenges in the heterogeneous catalysis and gas sensing fields.
Catalysts are substances that speed up reactions by providing an alternative pathway with lower activation energy than that required for the uncatalysed reaction. A lot of research, both fundamental and applied, has been carried out to investigate how catalysts work and to increase their efficiency.
The present work shows how the use of Dark Field Microscopy (DFM) coupled with surface plasmon spectroscopy, enables the direct observation of the kinetics of H₂ gas interaction with single gold nanorods (NR) coupled with Pt NPs. The plasmonic particles, gold NRs, act as optical probes, and enable the monitoring of the electron exchange through the measurement of their surface plasmon resonance (SPR) band shift. To improve the redox reaction kinetics, the Au NRs have been coupled with Pt NPs and embedded also into a TiO₂ low scattering matrix. Both the Au NRs and the Pt and the TiO₂ NPs have been synthetized by colloid chemistry. Several samples made of bare Au NRs, or Au NRs coupled with only Pt NPs or with Pt and TiO₂ NPs have been deposited by spin coating on silica substrates.
The longitudinal Au SPR band shift has been monitored by DFM looking at the variation of the scattering spectrum of a single Au NRs in the presence of H₂. Time-resolved measurements have been also conducted looking at the plasmon band shift, in wavelength, in order to monitor the kinetics of the H₂ reaction. With such measurements it was possible to elucidate the importance of the adsorbed oxygen and the TiO₂ matrix on the H₂ reaction with the Pt NPs.

Over the last several decades, the community of nanomaterial chemists have developed exquisite methods for creating gold nanostructures with controlled size, shape, and crystal form. As we move into the 21st century - "The Century of Biology" - biological applications of gold nanostructures abound, from chemical sensing to biological imaging to photothermal therapy. In this talk I will discuss some details in all these areas, and then show that the presence of gold nanoparticles in the extracellular matrix surrounding cells can lead to unanticipated effects; these effects could be used for good, if understood.

Bulk nanoporous gold (np-Au) provides a robust materials platform to develop new functional materials by surface functionalization and templating. Nanoporous gold is also an ideal platform to study size effects as the characteristic length scale of np-Au can be adjusted from a few nanometers to several micrometers without affecting the overall 3D architecture (porosity and connectivity of the ligaments) by applying a simple annealing procedure. Both surface functionalization and templating of bulk nanoporous Au require coating techniques capable of uniformly coating ultra-high aspect ratio structures (aspect ratio of 10⁴ for a 300 micrometer thick sample with 30 nm pores). This is the domain of atomic layer deposition (ALD) that in addition to uniformity provides atomic-scale thickness control by relying on self-limiting surface reactions. Our experiments reveal that even subnanometer-thick ALD coatings can stabilize the nanoscale morphology of np-Au up to 900 degree C while simultaneously improving the mechanical and catalytic properties. This, for example, opens the door for the development of gold-based catalysts for high temperature applications. ALD functionalized bulk nanoporous Au is also a very interesting platform to systematically study length scale effects in Li ion batteries. For example, it can be shown that by independently controlling pore size and the coating thickness of the active material (TiO₂ in our study) one can achieve profound performance improvements by matching the Li⁺ diffusivity in the electrolyte and the solid state through adjusting pore size and thickness/phase of the active coating. For thin TiO₂ films, supercapacitor like power performances can be realized.
Finally, by removing the Au core of ALD coated nanoporous gold, one can realize ultralow-density, ultra-high surface area bulk materials with interconnected nanotubular morphology. We demonstrated deterministic control over density (5 to 400 mg/cm³), pore size (30 nm to 4 micron) and composition. The materials are thermally stable and, by virtue of their narrow unimodal pore size distributions and thin-walled tubular design, ~10 times stronger and stiffer than traditional aerogels of the same density. Such low density foam samples open the door to more efficient (brighter) laser-induced nanosecond-scale x-ray (~ 5-10 keV) sources. Tests at the Omega laser have indeed revealed laser-to-x-ray conversion efficiencies between 5-10 % which are the highest conversion efficiencies that have been obtained from foam-based targets. The three-dimensional nanotubular network architecture also opens new opportunities in the field of filtration, and first experiments are currently performed and are very encouraging.
Work at LLNL was performed under the auspices of the U.S. DOE by LLNL under Contract DE-AC52-07NA27344. Project 13-LWD-031 was funded by the LDRD Program at LLNL.

Control over nanoparticle shape and size is commonly achieved via a seed-mediated approach, where nanoparticle precursors, or seeds, are hypothesized to template epitaxial growth. Despite the intimate relationship this suggests between seed structure and that of the final nanoparticle, structural control and characterization of the initial distribution of seeds are limited. We have found that an iterative reductive growth and oxidative dissolution process systematically controls seed structural uniformity. Further, we use this platform to study the implications of seed structure on: (1) the uniformity of gold anisotropic nanoparticles grown from them, (2) the ability to accurately determine physical properties of nanoparticles with well-established structures, and (3) the extent of ordering in nanoparticle crystallization. Importantly, this attention to seed uniformity improves noble metal nanoparticle uniformity and purity to unprecedented levels for eight different shapes produced from the same seed source, which enables particle optical extinction coefficient measurements and micron-size superlattice assembly.

Assembly of gold nanoparticles (AuNPs) into one-dimensional structure is known to allow plasmon tuning due to the plasmon coupling effect. We here report a systematic study on the synthesis, surface modification of charged AuNPs, their reversible assembly into one-dimensional chain structures and applications based on this plasmon tuning process. The thermodynamics of the assembly process is evaluated by analysis of colloidal forces among AuNPs. It is revealed the key to control the assembly behavior is manipulation of these colloidal interactions, which provided driven force for assembly/disassembly process while ensured reversibility. Kinetics of the assembly process is also characterized by SAXS while memory effect of the assembly process is also studied by delicate control of assembly kinetics. The plasmon tuning by this assembly/disassembly of AuNPs is also applied in different applications. The as-assembled AuNP chains are able to be blended in polymers to form composite films. When external pressures are subjected to the composite film, the displacement of AuNPs associated with the deformation of polymer caused the disassembly of the AuNP chains and result in the distinct color change of the film. Thus, the composite film can be applied as colorimetric pressure sensor. AuNPs can also be encapsulated in nanoshells by a template synthesis method and reversibly assembled and disassembled in a confined space. This color-tunable rattle-type nanostructure can be applied as a chemical sensor in localized environment.

Single Ag or Au nanocubes are known to be plasmonic nanoparticles with strong plasmonic fields which are concentrated around their corners. However, when these nanoparticles aggregate, they do so in a face-to face arrangement. The formation of hot spots between plasmonic nanoparticles in close proximity to each other is known to greatly enhance their plasmonic fields which is important in the field of imaging. Thus, what is the structural development of hot spots between two nanocubes in a face-to face dimer? Do they form between the corners or are they between the adjacent facets and what does this depend on? A detailed DDA simulation of Au-Au, Ag-Ag and Au-Ag dimers suggests that there is a competition between their formation in these two locations which depends on the inter-particle separation, the intensity, the wavelength and the polarization direction, of the exciting light (with respect to the dimer axis), It is also found that in the hetero dimer, the inter band electrons of the Au greatly quenches the plasmonic field of the Ag nanoparticles at short distances.

For the synthesis of highly porous gold structures, Au-Ag alloy is most suitable as starting material because it allows wet-chemical dealloying and has a complete solubility over the whole composition range [1]. Nanoporous Au is characterized by an extremely high surface-to-volume ratio, and exhibits unique properties for applications e.g. in optics, catalysis, sensorics, and energy-harvesting [2].
The porosity of the material after dealloying depends on the initial Au:Ag concentration and distribution of the atoms. For Au-Ag alloys it has been experimentally observed [3] and theoretically discussed [4] that at the nano-scale, various processes such as oxidation, phase segregation and surface diffusion influence the final Au:Ag composition and distribution in the porous nanostructures. Furthermore, the initial size and geometry of the object also affect the final morphology.
In this work, we report the investigation of the dealloying process of Au_1-xAg_x alloy nanowires in an acidic solution as a function of initial wire composition and dimension, studying in detail the surface morphology and composition of the nanostructures before and after dealloying. We synthesize cylindrical Au-Ag nanowires with controlled dimensions (diameter: 25, 45, and 80 nm, length ~ 30 µm) and different Au:Ag ratios (60:40 and 40:60 at. %) by electrodeposition from a single-bath electrolyte in the cylindrical pores of etched ion-track membranes. After deposition, the polymer membrane is selectively dissolved and the released nanowires are transferred onto transmission electron microscopy (TEM) grids. The selective dissolution of silver in nitric acid leads to rough and porous nanowires depending on the initial Au:Ag composition. To gain a detailed insight into the dealloying process we investigate size, elemental distribution and crystallographic properties of the nanowires before and after Ag dissolution. A compositional analysis with high spatial resolution is performed by means of energy dispersive X-ray spectroscopy at a high resolution TEM.
Our results evidence surface segregation effects in Au-Ag nanowires which are not only size dependent but also strongly vary with the initial wire composition. Furthermore, the results indicate that surface segregation occurs on a time scale of days (< 3 days) independently of the wire dimensions. We also show that we are able to obtain highly porous nanowires with silver content below 10% and ligament size from 5 to 30 nm.
[1] ASM Internationnal, “ASM Handbook, Vol. 3: Alloy Phase Diagrams,” in ASM Handbook, 1991.
[2] A. Wittstock, J. Biener, J. Erlebacher, eds. No. 22. Royal Society of Chemistry, 2012.
[3] D. Belic#769;, R. L. Chantry, Z. Y. Li, and S. a. Brown, Appl. Phys. Lett., vol. 99, no. 17, p. 171914, 2011.
[4] L. Wei, W. Qi, B. Huang, and M. Wang, Comput. Mater. Sci., vol. 69, pp. 374-380, Mar. 2013.

The ground-breaking discovery of surfactant templated growth of gold nanorods has inspired over a decade of research into the mechanisms that govern the shape of nanocrystals [1]. Pivotal to shape control is the crystallography and atomic stability of different crystal surfaces. However, measuring these quantities on surfaces that comprise just a small number of atoms presents a new challenge. Using a quantitative analysis of the intensity in atomic resolution scanning transmission electron microscope (STEM) images [2], we locate and count atoms in gold nanorods in a time sequence. This enables us to determine the atomic structure and relative stability of each of the facets in a nanorod and to correlate this with its overall shape and size [3]. We use this approach to measure and compare the dimensions and stability of low- and high- index facets co-existing in a gold nanorod [3]. We also consider the stability of the overall shape and facet crystallography, in the presence of significant movement of individual atoms from one atomic site to another [3]. This method provides insights into the relative surface energy of each of the crystal facets that are generated by, and collectively comprise, the overall chemical and geometrical environment of the nanorod
In this study, the gold nanorods were grown from small Au seed particles in the presence of cetyltrimethylammonium bromide (CTAB) and AgNO₃ [4] and the high angle annular dark field STEM was conducted using a double-aberration corrected Titan³ 80-300 FEG-TEM operating at 300kV.
[1] Yu, Y.-Y., Chang, S.-S., Lee, C.-L. & Wang, C. R. C. Journal of Physical Chemistry B101, 6661-6664 (1997);
[2] Dwyer, C., Maunders, C., Zheng, C.L., Weyland, M., Tiemeijer P.C., Etheridge J., Applied Physics Letters100, 191915 (2012);
[3] Katz-Boon, H., Walsh, M., Dwyer, C., Mulvaney, P., Funston, A.M., Etheridge, J., submitted (2014);
[4] Nikoobakht, B. & El-Sayed, M. A. Chemistry of Materials15, 1957-1962 (2003)

To date, so many excellent works have been reported for the applications of gold (Au) and silver (Ag) nanoparticles (NPs) to various fields. Meanwhile, combination of plasmonic NPs with functional polymers can further shed light on the characteristics of NPs. Here we will first present Au-hydrogel hybrid colloids as optical sensors in which multiple colors (maximum four colors: wine harr; violet harr; dark blue harr; faint blue) are switching reversibly in response to temperature changes. Spherical Au NPs are 2-dimensionally assembled on thermoreversible hydrogel NPs, and temperature-dependent swelling/shrinking behaviors of the hydrogel NPs in aqueous systems reversibly alter the assembly structures of Au NPs (packing density and interparticle distances) in the hybrid colloids. Accordingly, optical bandwidths of the hybrid colloids are reversibly changed with temperatures, and the bandwidth variations of the hybrid colloids are increased with the size of Au NPs (15 to 51 nm). With hybrid colloids assembled with 51 nm Au NPs, most optical signals are shown below 600 nm at room temperature while prominent optical signals are still recorded at 900 nm at 50 ^oC. Therefore, multiple colors are reversibly switched during repeated heating/cooling cycles.
Next, we will present the modification of Ag NPs&’ surfaces (nanocubes and nanospheres) with polyelectrolytes (either positive or negative) enables Ag NPs to use as probes for plasmonic-based, wide-range quantification of radical compounds. The quantification of radical compounds in a broad range with Ag NPs has been difficult due to nanoparticles&’ significant etching (high detection sensitivity) for a trace of radical. Through the modification, the polyelectrolytes protect the Ag NPs by probably either retarding (forming diffusion barriers) or preventing (blocking/entrapping/scavenging) the arrival of radicals to Ag NPs or both. The detection sensitivity (degree of etching) to Ag NPs by radicals and dominant mechanisms would be variable from the concentration and types of polyelectrolytes, respectively. The roles of the polyelectrolytes will be demonstrated by using radical compounds produced from tetrahydrofuran and H₂O₂. From the results, we could obtain calibration curves for the wide-range quantification of radical compounds. References: Lim et al., Chemistry of Materials 2014, 26, 3272; La and Cho, Analytical Chemistry 2014, 86, 6675.

Gold nanocages are nanostructures with hollow interiors and porous walls. The smallest nanocages that we have been able to prepare are only measured 12 nm in size. By controlling the wall thickness, their surface plasmon resonance peaks could be precisely tuned to any wavelength in the range of 400-1200 nm. In addtion to Au-197, we have also been able to prepare gold nanocages from radioactive isotopes including both Au-198 and Au-199. This gives us a new opportunity to fabricate nanostructures for multimodal imaging applications. In this talk, I will discuss recent developments in the synthesis of gold nanocages, as well as their novel applications in imaging, sensing, drug delivery, controlled rlease, and catalysis.

The discovery of metal nanoclusters (<3nm) has greatly expanded the horizon of nano-material research. These nanosystems exhibit molecular-like characteristics as their size approaches the Fermi-wavelength of an electron. The relationships between size and physical properties for nanomaterials are very interesting. Particularly, the changes in the optical properties have provided tremendous insight into the electronic structure of nanoclusters. The use of ultrafast laser spectroscopy on monolayer protected clusters (MPCs) in solution provides the benefit of directly studying the chemical dynamics of metal nanoclusters (core), and their non-linear optical properties. In this talk, the optical properties of gold and silver MPCs in the visible region are investigated using ultrafast spectroscopy. An emission mechanism for both the gold and silver nanoclusters is proposed based on both steady state and time resolved absorption and fluorescence up-conversion spectroscopy. We show that the nanoclusters behave differently from nanoparticles in terms of emission life-times as well as two-photon cross-sections. Further investigation of the transient (excited state) absorption has revealed many unique phenomena of nanoclusters such as quantum confinement effects and vibrational breathing modes. We have explored optical and electronic applications of these novel materials.

In the last years phosphine-stabilized gold clusters with defined molecular structures, such as the well-known undecagold cluster of composition [Au₁₁(PPh₃)₇Cl₃], have attracted increasing academic and technological interest. Their unique properties arise from size quantization effects, leading to nonmetallic properties, exhibited by a nonzero band gap (E_gap), which, in a first approximation, increases with decreasing cluster size and thus follows a simple scaling law. Hence, the ability to chemically access gold clusters with molecular precision and thereby to tune the electronic structure for the desired properties is of great importance for their applications in nanotechnology.
We have synthesized a series of triphenlyphosphine-stabilized gold clusters with a defined number of gold atoms, i.e. [Au₈(PPh₃)₈](NO₃)₂, [Au₉(PPh₃)₈](NO₃)₃ and [Au₁₁(PPh₃)₇Cl₃], in order to understand the interplay between structural features and the electronic structure. After crystallographic characterization of the full cluster structure including ligand shell, we assessed E_gap by means of optical extinction spectroscopy, photoluminescence spectroscopy, voltammetry and density functional theory (DFT) calculations. Furthermore, DFT calculations on our newly synthesized gold cluster [Au₁₄(PPh₃)₈(NO₃)₄] have been performed. A correlation of the characteristic energies derived from the experimental methods and DFT calculations revealed that the band gap does not follow a simple scaling law, but actually depends on the specific structural features of the clusters themselves as well as on charge effects of the cluster cores and the charge balancing counter ions.

The research on atomically precise monolayer-protected metal clusters has captivated the imagination of chemists, physicists and material scientists for the last ten years both for fundamental science and applications in catalysis, sensors and optoelectronics. Unique optical properties were observed from these clusters that show discrete electronic states and interesting luminescence properties. These properties are dependent on core size as well as on crystal structures. Our research efforts are focused on understanding the role of core-gold and shell-gold interactions on their optical properties. Ultrafast luminescence measurements have focused on understanding the relaxation of core-gold states while temperature-dependent optical properties were used to understand the interactions of core-gold and shell-gold. In this presentation, ultrafast luminescence and temperature-dependent optical properties of Au₁₄₄, Au₁₀₂ and Au₆₇ would be discussed. The results show that the structure and symmetry of clusters are crucial for understanding their optical properties. New results on ultrafast luminescence and temperature-dependent optical properties of Ag₄₄ and Au₂₂ would be presented that have shown contrasting behaviors.

Because of their molecular-like properties, ligand-protected gold nanoclusters have well-defined UV-vis or optical absorption bands. These bands are very characteristic and often used a fingerprint to identify different clusters. Therefore, an interesting question would be if the spin-orbit coupling could be reflected in the UV-vis absorption spectra of ligand-protected gold nanoclusters. A straightforward approach to this question is to compute or simulate the optical absorption spectrum for a specific cluster by comparing two models: with spin-orbit coupling and without. Then one compares the simulated spectra with the experiment. In this talk, we will discuss the effect of spin-orbit coupling on the optical absorption of the most widely studied thiolated gold nanocluster, Au₂₅(SR)₁₈^-. We will show that the ubiquitous shoulder peak or the double-peak feature below 2.0 eV of Au₂₅(SR)₁₈^-&’s optical absorption is due to spin-orbit coupling. We will further discuss the many implications of this finding.

Designing, synthesizing and controlling plasmonic nanostructures such as Au and Ag nanoparticles with high precision and high yield are of paramount importance in optics, nanoscience, nanobiotechnology and materials science. It is particularly important and challenging to generate and precisely control plasmonically coupled nanostructures with ~1-nm plasmonic gap for larger signal amplification and more quantitative signal output. Here, I will describe various functional molecule(e.g., DNA, protein, polymer, etc)-based synthetic strategies to build up new types of plasmonic nanoprobes with high structural controllability. The use of these plasmonic nanostructures including plasmonic nanogap structures and anisotropic nanostructures as excellent optical signal enhancement and biosensing platforms in detecting biomolecules sensitively, quantitatively and specifically will be mainly shown and discussed.

Nanoclusters are comprised of stable collections of small numbers of metal atoms (e.g. gold or silver, 2-144 atoms, le; 1.8 nm) that can have intriguing catalytic, magnetic or photophysical properties. Nanoclusters bridge the gap between atoms and nanoparticles, and exhibit unique physical and optical properties not seen in bulk materials due to quantum effects. Nanoclusters possess superatomic orbitals similar to atomic orbitals, where each atom of the nanocluster accounts for their properties, and where addition of an extra atom to a cluster completely changes its physical and optical properties.
Recently biomolecule templated metal nanoclusters have gained much attention due to their potential biological applications. Genetically encoded polymers (i.e. elastin like polymer (ELP)) consist of repeating units of sequence yielding a polymer with defined stimuli responsive, biocompatiblity and programed assembly. For the first time we report the direct synthesis of pure, single sized, stimuli responsive, water-soluble and biocompatible ELP templated Au nanoclusters. The polymer design, synthesis protocol, optical properties and electron microscopy and stimuli-response will be discussed.
Fluorescent nanoclusters, in particular DNA templated silver (Ag) clusters, have drawn much attention, as they have tunable fluorescence emission, single-timescale photoblinking, high quantum yields, and thus have large potential for applications in bio- imaging, sensing and for energy harvesting. Toward energy harvesting we have begun to assemble clusters, through DNA hybridization, and monitor their energy. We will discuss the synthesis, characterization, and photophysical properties of assembled silver nanoclusters.

Despite 20 years of progress in synthesizing thiolate-protected gold nanoclusters (or thiolated Au NCs), the knowledge of their growth mechanism and luminescence fundamentals still lags behind. Some of our recent work may shed light on these issues, and will be discussed in this presentation. In the first part, we will discuss the detailed growth process of thiolated Au NCs by tracing all stable intermediate species from reduction of Au(I)-thiolate complexes to evolution of thiolated Au₂₅ NCs (J. Am. Chem. Soc.2014,136, 10577). In the second part, we will discuss some fundamental issues on the luminescence properties of thiolated Au NCs, including the origin of emission and the size effect in luminescence. We will focus our discussion on a possible mechanism of aggregation-induced emission (AIE) of the Au(I)-thiolate complexes on the NC surface (J. Am. Chem. Soc. 2014,134, 16662). We will also extend the concept of AIE to a recently identified, highly luminescent thiolated Au NC, which has a precise molecular composition (Au₂₂(SG)₁₈) and structural information (J. Am. Chem. Soc. 2014,136, 1246).

Polynuclear hybrid Au(I) compounds exhibit a very large domain of applications such as electronic devices, contrast agents, sensors or photocatalysts. All these applications are related to the ability of gold(I) to form aurophilic interactions which give luminescent materials.[1] Among hybrid gold species, gold(I) thiolate compounds are an important class of materials due to the high affinity of gold for sulfur atoms that can generate molecular complexes, extended polymers, stabilized gold nanoparticles or self-assembled monolayers. Thus in nanoscience, gold(I) thiolate polymers are a key step in the Brust-Schiffrin synthesis of functionalized gold nanoparticles.[2] However relatively little is known about the relationship between the structure of the Au(I)-SR intermediate and the formation of the nanoparticles. In addition, the luminescence of gold thiolate nanoclusters, with gold core diameter less than 2 nm, is highly dependent of the structure, the arrangement and the length of Au(I)-thiolate motifs at the surface.[3] So to understand the formation of gold thiolate nanoparticles and clusters and also to explain the luminescence of gold thiolate coordination polymers and clusters, we will present the syntheses and the first structure resolutions, by powder X-Ray diffraction, of series of [Au(I)-SR]_n compounds. We will demonstrate that depending on the substituent (R = Ph, PhCOOH, PhNH₂) different structures are obtained: lamellar, helicoidal or cyclic. The photophysical properties governed by aurophilic inetractions and Ligand-to-Metal Charge Transfer are studied by room temperature and solid-state emission and lifetime decay experiments, and are explained by DFT calculations. Among those coordination polymers, two exhibit dynamic OFF/ON emission switching, one associated with an irreversible solid-state thermal crystallization and the other one from a reversible esterification. In addition we will show, from those preformed coordination polymers, that exchange reactions of thiol ligands involve dissolution/recrystallization mechanism, pointing out that each thiolate ligand induces its own polymeric structure.
REFERENCES
[1] Schmidbaur H., Schier A., Chem. Soc. Rev., 41, (2012), 370.
[2] Lavenn, C.; Albrieux, F.; Tuel, A.; Demessence, A., J. Colloid Interface Sci., 418, (2014), 234.
[3] Luo Z., Yuan X., Yu Y., Zhang Q., Tai Leong D., Yang Lee J., Xie J., J.Amer. Chem. Soc., 134, (2012), 16662. Das A., Li T., Li G., Nobusada K., Zeng C., Rosi N. L., Jin R., Nanoscale, 6, (2014), 6458.

Use of nanostructures in various applications such as biodetection, catalysis and drug delivery has led to the need for faster and more controlled synthesis routes. Recent advances have been made which allows the production of nanostructures with control over particle size, shape, and composition. These architectural parameters dictate the resulting physical and chemical properties. In particular the extraordinary optical properties of such nanostructures can be used in a variety of highly sensitive chemical and biological detection schemes, and may also find use in novel, nanoscale optoelectronic and photonic devices. The majority of the recent efforts has focuses on isotropic, pseudo-spherical structures using chemical synthetic routes. We report a simple thermal chemical vapor deposition route to produce single-crystal Au nano-crystals without using any external catalysts. Heating AuCl₃ precusor in a furnace above 450C in an argon environment with Si (100) substrates leads to the formation of Au nanostructures like prisms, triangular plates, decahedrons, icosahedrons and hexagons which grow vertically on the Si substrates. We observe a fine interplay of reduction of surface energy vs internal stress, which plays an important role in the morphology of gold nano-crystals. The strain distribution is influenced by the Au-substrate interface and Au-air interface, which arise due to differences in their thermal expansion. We have studied the growth and properties of singe crystal anisotropic gold nano-crystals by utilizing synchrotron based coherent diffraction imaging to study these nanostructures along with other traditional characterization techniques like XRD, EDX and AFM. We find the lattice strain at the surface of the octahedral nanocrystal agrees well with the predictions of the Young-Laplace equation quantitatively, but exhibits a discrepancy near the nanocrystal-substrate interface resulting from the interface. Application of this approach to the growth of other metallic nanostructure will be discussed.
Research at UCSD support by NSF DMR Award # 1411335.

Gold nano-clusters, owing to their exceptional chemical, optical, and electronic properties, hold promise in a wide range of applications, such as optoelectronics, bio-recognition, and catalysis. Interestingly, Au nano-clusters are known to exhibit a diverse variety of structural configurations including planar, hollow cages, chiral tubes, and bulk-like pyramids or icosahedra. Despite these earlier observations, a fundamental understanding of the atomic structure of Au nano-clusters is still lacking - in particular, dependence of the structure of an Au_n cluster on its size n (i.e., number of atoms). For instance, there is an open debate on the critical cluster size n_c beyond which globular isomers become energetically preferable over planar ones, with reported values of n_c ranging from 7 to 15. Furthermore, the knowledge of structural transitions between energetically close isomers (~20 meV/atom) at a given size is still in its infancy. Global optimization and molecular dynamics (MD) simulations via empirical force fields (EFFs) provide an ideal route to address these issues. However, none of the available EFFs can adequately capture the dimensionality effects from bulk to sub-nanometer clusters. Popular pairwise Au EFFs, such as Morse, or those that account for multi-body effects via an embedding function (e.g., EAM, MEAM) tend to artificially over-stabilize bulk-like isomers. We introduce a new hybrid EFF wherein the short-range Au-Au interactions are described by a bond-order based Tersoff potential while the long-range Au-Au attractions (called aurophillicity) are accounted via a Sutton-Chen like multi-body embedding function. We employed genetic algorithms to parameterize a hybrid EFF of Au by training against cohesive energies of representative clusters and condensed phases of Au computed by density functional theory (DFT) calculations. Our structural searches using this new EFF showed that such a hybrid functional form is capable of describing the diverse range of structures of Au nanoclusters consistent with previous reports, e.g., planar structures circa Au₁₃, hollow cage at Au₁₉, and space-filled structure at Au₅₅. Furthermore, an atom-by-atom formation of Au₁₃ via long time MD simulations using our hybrid EFF results in a configuration identical to the global energy minimum structure predicted by DFT. These results indicate that the structure of gold nanoclusters is governed by the interplay of short- range bond-order based Au-Au interactions and long-range aurophillic attractions. Finally, our new hybrid EFF, which can describe Au very well over sizes from bulk down to sub-nanometer (few-atoms), will open doors to structure prediction of technologically relevant mid-size Au clusters (n=20—80) that are not tractable by DFT alone.
Use of the Center for Nanoscale Materials was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357.

Molecular level coupling between the organic canopy (ligand) and metal nanoparticle determines both the physical strength of the interface, as well as the optoelectronic response arising from molecular orbital-plasmon hybridization. Traditional gold nanorod functionalization utilizes secondary (e.g. pyridine metal chelates, induced electrostatic dipoles (CTAB, SDS)) or weak covalent (Au-thiol) bonds. Although these facilitate facile surface modification via ligand exchange, they result in a “dynamic” interface that is in equilibrium with a finite concentration of free ligand, not stable under mild processing conditions, and permeable by oxidants. Furthermore, a molecular junction via linkers such as thiol and amine may obstruct interfacial electron transfer and foster weak molecular orbital-plasmon hybridization; potentially limiting performance in electronics, sensors, or data storage. Here in, we quantitatively demonstrate by Raman and UV-Vis spectroscopy that diazonium salts produce a stable Au-carbon bond on AuNRs. This motif has a higher binding energy, and thus is more robust and less prone to oxidation. The ligand architecture can be tuned with reaction conditions to selectively favor the ends of the AuNRs, as well as producing a single layer or multilayer aryl canopy. The terminal groups of the aryl can be further modified by a simple redox reaction, enabling derivatization into a variety of optically or biologically sensitive moieties such as azobenzene and peptide units. This facile diazonium grafting to Au NRs provides opportunities to exploit the rich array of recently developed diazonium functionalization chemistries for carbon nanostructures, and thereby opening a new generation of surface modifiers and coupling agents to form a wider range Au NRs compounds for optoelectronic applications.

There are two central issues regarding polymer electrolyte membrane fuel cells (PEMFCs) before they are considered viable for practical applications: poor catalytic performance of a pure platinum cathode catalyst for the oxygen reduction reaction, and the propensity for a pure platinum anode catalyst to become poisoned and rendered inactive. For this reason, bimetallic Pt/M or M/M and trimetallic P/M/M nanoparticles, where M is a cheaper metal, with higher catalytic activity are desirable. A block copolymer templating method is presented to yield efficient nanoparticle catalysts for implementation into PEMFCs. Three different Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers of block ratio ~3:1 were used to synthesize the nanoparticles. The templating method was used to control size, morphology, structure, and composition. A full study was done on a platinum/gold system, utilizing cyclic voltammetry coupled with a rotating ring-disk electrode, x-ray photoelectron spectroscopy, and transmission electron microscopy. Studies are still underway on both a palladium/gold system and a Pt/Pd/Au system, which will be characterized via scanning force microscopy, laser ablation inductively coupled plasma-mass spectrometry, transmission electron microscopy, and cyclic voltammetry coupled with a rotating ring-disk electrode. The catalysts were all subjected to activity testing for the oxygen reduction reaction, which took place at a fuel cell cathode, and the kinetics were calculated with favorable results.

A facile and green one-pot synthesis of highly luminescent gold nanoclusters was recently developed where protein molecules act as the protecting ligand and as the reducing source for gold atoms [1]. Only a few years after this discovery, many other proteins or biomolecules were identified that can yield similar luminescent gold nanoclusters with a variety of potential applications from toxic species detection to bioimaging [2]. Not only is this synthetic methodolgy useful for the fabrication of biocompatible nanosensor devices, it also demonstrates the utility of the biomimetic or protein-directed synthesis of gold nanostructures as a cost-effective, clean and controllable process. The detailed mechanism underlying gold nanocluster formation and the local structure of such highly luminescent gold nanoclusters within the protecting protein has been elusive for many years. Towards this end, we report a time-dependent synchrotron-based X-ray absorption spectroscopy study on the formation of luminescent bovine serum albumin-protected gold (BSA-Au) nanoclusters. Gold nanoclusters appear to form via a gradual reduction pathway from Au³⁺ to Au⁺, associated with a transition from Au³⁺-Cl to Au³⁺-OH bonding and then finally to Au⁺-SR structures. An in-depth multi-shell extended X-ray absorption fine structure (EXAFS) fitting analysis of the luminescent BSA-Au nanocluster product revealed interlocked Au⁺-SR rings as the main structural component within the BSA scaffold. Additional studies investigate the fluorescence quenching mechanism of Hg²⁺ ions in solution and the gold nanocluster formation mechanism for other protecting proteins such as lysozyme.
[1] Xie et al. J. Am. Chem. Soc. 2009, 131, 888-889
[2] Chevrier et al. J. Nanophoton. 2012, 6, 064504

In recent decades, plasmonic metals, such as gold and silver, have been investigated in various fields for electrical devices, sensors and surface enhanced Raman spectroscopy due to their strong surface plasmon resonance (SPR). New physicochemical properties can be achieved by controlling the shape, size and component of materials. However, common methods of the nanoparticle synthesis are difficult to control the shape and have limits the geometry of the as-synthesized materials. To circumvent these restrictions, we used silica cage for a synthesis of nanoparticles. Silica cage, having porous structure, is chemically and physically stable, and which can be applied to the synthesis template. As this method, we can easily synthesize gold/silver bimetallic nanoplate in tailored silica cage by galvanic reaction. By transmission electron microscopy and fast Fourier-transform (FFT) pattern analysis, we can obverved that gold and silver atoms were partially arranged in the bimetallic structure and formed grain boundaries; gold is cubic lattice and silver is hexagonal lattice. While, the nanoparticles were randomly grown and aggregated without silica cage.

We present a method to follow kinetics of silver overgrowth of gold nanorods in-situ. Starting from synthesis of well-defined gold nanorods with low amounts of impurities, narrow size distribution and high gold ion conversion surfactant based silver overgrowth kinetics are revisited. Combining careful analyses of static experiments and modeling of optical spectra enables us to follow overgrowth kinetics in-situ with UV/Vis/NIR spectroscopy and compare the influence of two different surfactants on kinetic parameters. CTAC and BDAC have the same counterion and thus stabilize the same crystalline facets. However, they show significant differences in reaction rates due to the different headgroups BDAC overgrowth is about one order of magnitude slower than CTAC, which can be directly correlated to the surfactant headgroup structure.

Over the last decade, many different methods have been developed for the synthesis of noble-metal nanocrystals with controlled sizes, morphology, and compositions. Seeded growth represents one of the most powerful approaches but its full capability involving two noble metals is still limited by galvanic replacement reaction (GRR). In this talk, we will present two routes to the growth of Au on Ag nanocubes without GRR. In the first approach, we introduce a strong reducing agent to rapidly reduce Au³⁺ ions before they undertake GRR with the Ag nanocubes. When the fast reduction progresses without any homogeneous nucleation in the growth solution, Au atoms nucleate and grow onto the surfaces of the Ag nanocubes to generate Ag@Au core-shell nanocubes with enhanced SERS properties and improved chemical stability. In the second approach, we include Ag⁺ ions in the growth solution to effectively surpass GRR. In this case, both Ag⁺ and Au³⁺ ions are co-reduced by a reducing agent to form atoms for their selective deposition to the corners and edges of Ag nanocubes, leading to the formation of Ag@Au core-frame nanocubes. Upon removal of Ag element, Ag-Au core-frame structures exhibit catalytic properties.

We synthesized water-soluble and fluorescent Ag nanoclusters (NCs) stabilized by glutathione at room temperature. Glutathione (GSH) was introduced to increase the stability and water solubility of the Ag NCs. Meanwhile, with this facile approach to synthesis of Ag NCs have surprisingly high efficiency of selective Hg²⁺ sensing, where the limit of detection (LOD) was as low as 10^-10M (0.02 ppb,0.1 nM).This result indicated that using fluorescent Ag NCs as sensor probe to detect Hg²⁺ is a simple and practical strategy.¹ Then using the presynthesized and size-controlled Ag NCs as templates, we synthesized fluorescent gold nanodots (GNDs) with strong fluorescence (quantum yields sim; 10%), high stability and surface bioactivity by the application of galvanic replacement method.² The as-prepared GNDs have perfect photoluminescence properties with high photo-, time-, metal-, and pH- stability. This is because that the GSH as the ligand protect the GNDs and the presence of Au(I)-S complexes on the surface of the gold core. GSH make GNDs with excellent water solubility and favorable biocompatibility because it is a naturally and readily available tripeptide with carboxyl and amino functional groups. These advantages, combined with their small size, indicate that the as-prepared GNDs have potential application in biological labeling.

In this study, temperature and pH responsive cationic pentablock copolymers were used for the first time in the development of polyplex and gold nanoparticle (AuNP) based multicomponent (MCS) siRNA delivery systems. The pentablock copolymers consisting of Pluronic F127 (poly(ethyleneoxide)-block-poly(propyleneoxide)-block-poly(ethyleneoxide)(PEO-b-PPO-b-PEO)) and PDEAEM (poly(2-diethylaminoethyl methacrylate) blocks were synthesized through atom transfer radical polymerization. The polyplex systems were developed through the electrostatic interactions between the positively charged copolymers and negatively charged siRNA by varying the N/P ratio, while the multicomponent systems composed of AuNPs, siRNA and copolymers was produced by subsequent deposition of components on gold core using cleavable disulfide bonds and electrostatic interactions. The performances of the systems against luciferase-expressing SKOV3 ovarian cancer cells were evaluated in terms of siRNA protection, stability, toxicity, cellular uptake, luciferase expression suppression and intracellular distribution. The results indicated that in the presence of RNase enzymes and serum proteins, the systems did not show any dissociation and managed to protect siRNA from degradation. The cell uptake results and confocal images demonstrated that the micellization of free Pluronic F127 blocks enhanced cellular uptake while free tertiary amine groups in the PDEAEM blocks enabled the proton sponge effect and subsequently endosomal escape resulting in cellular accumulation for both systems. The MCS systems with 100 nM of siRNA loading showed 60% decrease in luciferase expression, whereas polyplex systems with same siRNA loadings managed to achieve 45% decrease without severe toxicity. The cleavable disulfide bonds in MCSs allowed sufficient release of the siRNA in the cytoplasm leading to better transfection efficiency of the MCSs compare to the polyplex system. The overall results suggest that the advantage of each block in the pentablock copolymer is utilized better in the multicomponent conjugate system. In addition, small loadings of siRNA in the MCS systems require small amount of polymer for efficient siRNA condensation, resulting in a non-toxic delivery system. The molecular weight of the cationic blocks was found to be critical for cell viability. An increase in molecular weight of the overall copolymer from 26 to 36 kDa, due to increase in the lengths of the cationic PDEAEM blocks, keeping the lengths of the non-ionic blocks constant, significantly increased toxicity of both polyplexes and MCS on the SKOV3 cells. This provides a facile way to tune the cytotoxicity of the pentablock copolymers to achieve efficient siRNA delivery, while maintaining biocompatibility. It can be concluded that the responsive pentablock copolymers are promising vectors to deliver siRNA, especially when used in AuNP based MSCs, however, their potential use should be further evaluated in vivo.

Surface Enhanced Raman Scattering (SERS) is a very valuable technique for probing the mechanisms and kinetics of catalyzed reactions in situ, since SERS gives detailed molecular information specifically from species that are right at the surfaces of metals. These types of in situ experiments not only lead to a greater understanding of the catalyzed reactions, but also to increased knowledge about the role of the catalyst in such reactions. Significant challenges occur since nanoparticles are needed that are (i) highly SERS active, (ii) very stable and (iii) highly catalytic. Highly SERS active particles are noble metal nanoparticles that have diameters in the range of 10 nm-1000 nm. Very stable metal nanoparticles tend to have thermodynamically stable low-index facets and are often made from gold. Highly catalytic particles have surfaces with many high-index facets or surface defects such as those found in nanoparticles with diameters of less than 10 nm. In order to accommodate these somewhat contradictory requirements for successful in-situ SERS studies, we have prepared dual-function nanoparticles that consist of Au-decorated truncated Ag nanocubes with diameters of 40 nm. These dual-function nanoparticles (i) give significant SERS signals with incident lasers of wavelength 785 nm and (ii) catalyze reactions, e.g. the reduction of 4-nitrothiophenol by sodium borohydride, at room temperature. Using dual-function particles, rather than mixing or combining small catalytic nanoparticles with large SERS active nanoparticles, simplifies the in situ studies. When using dual-function nanoparticles in a reaction solution, there are no substrates such as glass slides to consider when elucidating the mechanism, and there are no concerns about the reproducibility and uniformity of mixtures of two types of nanoparticles on a substrate. We will report on our ongoing in-situ experiments using these gold and silver nanoparticles, including detailed kinetics and mechanistic studies of the catalyzed reactions.

Graphene decorated with well-defined gold nanocrystals form functional nanocomposites that have many potential applications in electronics, optical sensors and Raman signal enhancers. These materials are commonly produced in direct reduction of gold precursors like HAuCl₄ or KAuBr₄ by graphene. However, in order to reduce surface energy, this generally forms approximately spherical, zero dimensional, gold nanoparticles. For tailorable electrical and optical properties, two dimensional gold structures such as nanoplates are usually obtained with the assistance of templates and surfactants. Unfortunately, these processes are inefficient, complicated and time-consuming.
Here, we demonstrate an in-situ, efficient and surfactant-free synthesis of ultra-thin (~20nm) and high aspect ratio (50:1) gold nanoplates. The synthesis includes two steps: (1) oxidize graphene surfaces with atmospheric pressure plasma for ~30 seconds and (2) directly reduce KAuBr₄ solutions with the activated graphene for two hours. Experimental results from EDS, TEM and SAED show that the plates are pure gold and have a single crystalline FCC structure with{111} oriented basal planes. We also show that the Raman scattering signal from gold nanoplates deposited graphene was significantly enhanced by 10-fold.

Well-defined micron- and nanoscale patterns are essential components toward multiplexed lab-on-a-chip technologies with spatially encoded optical, electronic, mechanical, and chemical properties. Chemical lift-off lithography can efficiently and reusably transfer ultrathin (< 1 nm) gold onto transparent and otherwise featureless polydimethylsiloxane (PDMS) substrates with user defined patterns over millimeter areas. Chemical lift-off lithography ‘lifts-off&’ molecules from a hydroxy-terminated self-assembled monolayer (SAM) on a patterned gold master (100-nm-thick gold features on silicon) onto an activated PDMS stamp. We present a strategy to use this subtractive mechanism to pattern ultrathin gold over millimeter areas with pattern features having lateral dimensions spanning tens of nanometers to millimeters. Elemental analysis via X-ray photoemission spectroscopy confirms the presence of gold, while atomic force microscopy show that the lifted-off layers, comprising both the SAM molecules and gold species, are 1-2 nm high, indicating the gold may be monatomically thick. This patterning technique is chemically selective; beginning with a SAM on gold with alternating lines of hydroxy and methyl termination yields the same pattern as functionalizing and lifting-off from patterned gold. Because the gold masters can be reused for multiple cycles of SAM formation and chemical lift-off to pattern many PDMS substrates, this patterning strategy is a route to combining the chemical functionality and optical properties of ultrathin gold with PDMS, enabling investigations of fundamental chemical and physical interactions between molecules and gold surfaces. Because PDMS is abundant, biocompatible, and therefore the basis of many flow cell technologies, these patterns can potentially be incorporated into chemical and biological optical microscopy tools.

Silver nanocubes have strong electromagnetic field enhancements at their sharp corners, which can drastically increase the Raman scattering cross sections of molecules at these sites for surface-enhanced Raman scattering (SERS). However, the susceptibility of elemental Ag to oxidation often leads to corner truncation and thus deterioration of SERS activity. In this talk, we report a robust synthesis of Ag@Au core-shell nanocubes by directly depositing Au atoms on the surfaces of Ag nanocubes as conformal, ultrathin shells. Our success relies on the introduction of a strong reducing agent to compete with and thereby block the galvanic replacement between Ag and HAuCl₄. By controlling the amount of HAuCl₄ injected, we can control the deposition Au on Ag cube form 3 to 6 atomic layers. An ultrathin Au shell was able to protect the Ag in the core in an oxidative environment. Significantly, compared to the original Ag nanocubes, the SERS activity of Ag@Au nanocubes with 3 atomic layers showed about 5-fold enhancement owing to an improvement in chemical enhancement. Interestingly, the SERS properties of Ag@Au nanocubes can be tuned by controlling the different Au layers on Ag nanocubes. When the thickness of Au layer was increased from from 3 to 6 layers, we found that the SERS would gradually decrease.

Metal nanotubes have been under investigation due to their characteristic structure and usage in electrical devices, sensors, catalysts and also medical applications. Among the metals, gold nanotubes attract much interest because of their lower toxicity, catalytic, electrical and optical properties with a remarkable resistance to oxidation.
Several method have been used to produce nanostructure for instance chemical etching, galvanic replacement reactions and many other methods based on the template approach such as electrochemical deposition sol-gel, chemical vapor deposition and electroless plating. In this work, a new route is introduced to synthesis single crystalline, defect free and freestanding Au nanotubes from silver nanorod template by the dissolution of the core material by the grain boundary diffusion process. For this aim, single crystal Ag nanowhiskers were grown on a carbon-coated Si/SiO₂/Si₃N₄ substrate at 650°C and subsequently coated with thin Au film employing a molecular beam epitaxy method. The Au forms a single crystalline epitaxial layer on the NWs and a polycrystalline Au film on the substrate surface. Afterwards, as prepared core-shell structure were annealed at 170°C in a rapid thermal annealing furnace for different times. The microstructures were investigated by high-resolution scanning and transmission electron microscopy, respectively. The distribution of Ag in the vicinity of the NWs both along and across the Au polycrystalline film was determined with the energy dispersive X-ray spectroscopy (EDS). The cross-section EDS analysis of Au film in the vicinity of NW demonstrated Ag enrichment at the surface of Au film and at the Au-carbon interface. An inverse dependence of the hollowing rate on the NW diameter was determined. We developed a quantitative model of the hollowing process which takes into account all relevant diffusion processes along the surfaces, interfaces, and grain boundaries. The predictions of the model are in a good agreement with the experimental findings.

Gold cluster - peptide nanoprobes have attracted increasing attention in life science and biomedicine fields. Especially, gold cluster - peptide nanoprobes exhibit the promising application for early disease diagnosis because of their sensitivity and stability in biomedical targeting detection. However, there exist the bottleneck in the nanoprobe design and construction. As we known, the impact of gold cluster is still impossible to be considered in the targeting peptide screening experiments (phage display). This limits the nanoprobe targeting validity badly when the selected peptide sequence is conjugated by gold cluster to compose the nanoprobe. The targeting peptide sequences could even lose their targeting binding totally after being conjugated by gold cluster. To meet the practical challenge, we utilize molecular simulation methods including density functional theory (DFT) and molecular dynamics (MD) to construct and design the nanoprobe, in which both the gold cluster and peptide could be directly considered as a complex in the initial simulation modeling. Moreover, the impact of gold cluster to the interaction of targeting peptide sequence can facilitate the development of nanoprobe by designing possible targeting sequence and suggesting the way of gold cluster to avoid neglect influence at molecular level. In this report, we identify the sequence based on γC peptide which can specifically target to integrin alpha_IIbbeta₃. γC peptide is the C-terminal part of fibrinogen and it is recently found as the native binding ligand to integrin alpha_IIbbeta₃. The traditional targeting sequence of integrin is RGD (Arg-Gly-Asp) circular peptide. While, the RGD peptide can bind to various integrin. To our knowledge, there is not a probe can specifically target unique integrin. In this report, we design the novel gold cluster conjugated nanoprobe based on γC peptide, which could effectively target to integrin alpha_IIbbeta₃ uniquely. In addition, the designed γC-like sequence is then mutated to improve solubility and retain critical binding stability. From the binding mode and binding affinity study of such designed targeting sequence conjugated by gold cluster, we find the conjugation of gold cluster to the N-terminal part of peptide does not affect the targeting binding to integrin which is largely attributed to the residues in C-terminal part. Our designed nanoprobe is fabricated experimentally. Based on the confocal fluorescence co-located images and the AFM single molecule force spectroscopy results, we verify that the novel nanoprobe could specifically targeting integrin a_IIbb₃ with enough stability. As a result, the theoretical design principle and targeting molecular mechanism could be extended for a series of novel nanoprobe developments. We bring a new insight to the gold cluster - peptide nanoprobe design from theoretical viewpoint, which is helpful for the targeting detection research in live cells.

Materials which have strong electron-electron interactions, so-called strongly correlated materials, are known to display exotic behaviours. Examples of such materials include high temperature superconductors and colossal magnetoresistive manganites.
Recently, we have found that molecularly-linked gold nanoparticle films represent a new class of materials that exhibit correlated electronic behaviours. Most notably, (a) they undergo a percolation insulator-to-metal transition as a function of film thickness, (b) as the system transitions from an insulator to a metal, a previously unobserved zero-bias conductance peak emerges, attributed to electron correlations, and (c) Coulomb effects play an important role in the conductance on both the insulating and metallic sides near the transition.
Specifically for the latter case, near the insulator-to-metal transition on the insulating side, conductance (g) behaves as g = exp[-(T_o/T)^1/2], where T_o is the absolute temperature. This behaviour, first predicted by Efros and Shklovskii, describe hopping transport in a system with significant Coulomb charging barriers. On the metallic side, we have found that conductance goes linearly with T^1/2. This T^1/2 correction term has been predicted by Altshuler and Aronov for a Fermi liquid with disorder mediated electron-electron interactions. Remarkably, the temperature correction factor is large and fitting conductance to g = g_o + aT^1/2 yields anomalously small elastic scattering lengths. The scattering length is found to be smaller than inter-atomic Au-Au separation, violating the Ioffe-Regel limit and signalling strong electron-electron interactions.
These remarkable results show that gold nanoparticle films can serve as a new test bed for studying correlated electrons as well as the promise of a nano building-block strategy for fashioning novel correlated materials.

Plasmonic nanostructures of noble metals (e.g., gold and silver) exhibit localized surface plasmon resonance which causes significant scattering and absorption in the visible spectrum. Theoretical studies show that the resonance band position of the plasmonic nanostructures can be shifted by engineering either the interparticle distance or the dielectric environment. In addition, the intensity of the plasmonic peaks can be tuned by aligning the anisotropic plasmonic nanostructures. By taking advantage of these dependences, we have developed various stress-responsive colorimetric films by blending the plasmonic nanostructures into polymeric materials. The mechanical stress stimulates the plasmonic shift, causing the color change, which in turn provides opportunities to correlate that stress with the color. By further controlling the nanoarchitectures of the plasmonic materials, we are able to make colorimetric pressure sensors withstanding wide ranges of threshold pressures. Those pressure sensors can be used to monitor the stress between two contacting or impacting surfaces by outputting color information.

A key issue in the use of nanomaterials is controlling how they interact with themselves and with the outer world. Our research program focuses on the tailoring of nanoparticles of surfaces for a variety of applications, coupling the atomic-level control provided by organic synthesis with the fundamental principles of supramolecular chemistry. Using these engineered nanoparticles, we are developing particles for biological applications, in particular delivery and sensing. This talk will focus on the interfacing of nanoparticles with biosystems, and will discuss our use of nanoparticles for delivery applications including our use of Nanoparticle-Stabilized Nanocapsules (NPSCs) for direct delivery of proteins to the cytosol. This presentation will also feature the use of nanoparticles for diagnostic applications, including the use of array-based sensing paradigms for the detection and identification of cancer.

Nanoparticle functionalization with peptides has proven to be a good strategy to inhibit particle aggregation, increase nanoparticle solubility and develop nanocarriers and biosensors [1,2]. Cell-penetrating peptides, such as, the transcription transactivation TAT peptide from human HIV-1 virus, have become a popular choice for transfection and other types of cellular delivery [3]. Studies have shown that TAT-peptide functionalized gold nanoparticles (AuNP) have the ability to penetrate cell membranes and localise in the nucleus [4,5], making them potentially a good system for drug-delivery applications, albeit with variable performance.
Detailed investigation of the effects of peptide concentration and distribution in the functionallayer at all-atom resolution can aid the design of gold nanomaterials for efficient drug delivery. With this in mind, we have performed an experimental and computational study on the effects of TAT peptide concentration on the structure and dynamics of the peptide layer on the 3 nm AuNP surface and its impact on the NP's membrane permeation capacity [6].
We show that cell internalisation of TAT functionalised nanoparticles is strongly dependent on TAT surface concentration and distribution. We present the mechanism by which TAT may become inactive upon surface immobilisation and demonstrate how the local environment of the functional peptide grafted onto the nanoparticle surface may affect properties essential for efficient cellular uptake. We found that a notably increased membrane activity correlates with the peptide backbone rigidity and increased basic residue solvent exposure attained at a particular concentration range of the functional peptides. The theoretically predicted membrane permeating capacity was confirmed experimentally while we also revealed molecular properties relevant to the experimentally observed inactivation with increasing TAT concentration. Our results suggest that nanoparticles may be engineered to specifically permeate membranes by targeted conjugation of peptides at specific concentration and surface distribution.
[1] P.D. Howes, R. Chandrawati, M.M. Stevens “Colloidal nanoparticles as advanced biological sensors” Science 346:1247390 (2014)
[2] A. Makarucha, N. Todorova, I. Yarovsky “Nanomaterials in biological environment: a review of computer modelling studies” Eur. Biophys. J, 40:103 (2011)
[3] N.A. Brooks, et al. “Cell-penetrating peptides: Application in vaccine delivery”, Biochem. Biophys. Acta, 1805:25 (2010)
[4] Z. Krpetic, et al. “Negotiation of intracellular membrane barriers by TAT-modified gold nanoparticles.” ACS Nano 5:5201 (2011)
[5] J.M de la Fuente, C.C. Berry “Tat peptide as an efficient molecule to translocate gold nanoparticles into the cell nucleus”, Bioconjugate Chem. 16:1176 (2005)
[6] N. Todorova, et al. “Surface presentation of functional peptides in solution determines cell internalization efficiency of TAT conjugated nanoparticles” Nano Lett. 14(9):5229 (2014)

In this talk, I will review the interactions that take place when ligand-coated gold nanoparticles interact with various types of viruses. First the interactions that can take place will be evaluated, then the effects of these interactions will be presented. In particular what we be discussused is the possibility for a nanoparticle to stabilize a vaccine viral vector and the possiblity for a nanoparticle to de-stablize a virus so to fight its infectivity. General conclusions will be drawn and design principles will be presented.

Quantifying proteins on a single cell level would promote studies of the role of cellular heterogeneity in disease progression and response to drugs. Clinically, accurate proteomic analysis of rare cells, for example, circulating tumor cells would contribute to early disease detection and monitor the prognosis of patients. Therefore, reliable, sensitive, accurate protein quantification methods at single cell resolution are in great demand. Although some recently developed methods provide possibility for single cell analysis, the quantification of proteins is not so accuracy. For example, one attractive approach is mass cytometry. In this method, the abundance of the proteins is assessed by the number of lanthanides conjugated to each antibody. Since the amount of lanthanide conjugated to the antibody is not precisely controlled, the quantification may be rough.With the advantage of easy sample preparation and high sensitivity, Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been used to quantify proteins in recent years. In these reports, the principle of protein quantification is the same as the one in mass cytometry. Therefore, the quantification is still not accurate as mass cytometry.
In this report, we use our peptide-Au cluster probes to realize single cell in situ protein precisely analysis. We firstly utilize the fluorescent property and targeting ability of our probes to recognize specific cell membrane proteins successfully, and subsequently immunofluorescence technique and blocking studies further confirm the specific membrane proteins are targeted by the probes. As the formula of our probe is clear, e.g. one probe is with 24 gold atom, with the assistance of single cell LA-ICP-MS methods, we can get the precise protein level at single-cell resolution. Integrins belong to cell membrane proteins, which mediate cell-cell adhesion events and are essential for the attachments of cells to extracellular matrices. Up until now, integrin αvβ3 has been widely studied because of its correlation with tumor growth and metastasis. Human erythroleukemia cell line (HEL), which expresses integrin αIIbβ3, was chosen as a model. We believe that to disclose the precise number and αIIbβ3 at single cell level will greatly help researchers to study erythroleukemia cancer progression.

Silver nanocrystals have fascinating optical properties known as localized surface plasmon resonance (LSPR), which is essential to applications in surface-enhanced Raman scattering (SERS), optical sensing, and biomedical imaging. Silver nanocubes, in particular, have strong electromagnetic field enhancements at their sharp corners, which can drastically increase the Raman scattering cross sections of molecules at these sites for ultrasensitive detection and imaging. However, the susceptibility of elemental Ag to oxidation often leads to corner truncation and thus deterioration of SERS activity. One potential solution to improve the chemical stability of Ag nanocrystals is to form alloys with a more stable metal such as Au. In this talk, I will discuss a new approach to the fabrication of Ag@Au core-shell nanocubes with superb SERS activity and chemical stability. The success relies on the introduction of a fast reduction by ascorbic acid to compete with and thereby block the galvanic reaction. An ultrathin Au shell was able to protect the Ag in the core from dissolution in an oxidative environment. Significantly, the core-shell nanocubes exhibited LSPR properties essentially identical to those of the original Ag nanocubes while the SERS activity showed an increase owing to the improvement in chemical enhancement.

Conjugating small concentrations of gold nanoparticles to the nucleus membrane of the cancer cells we were able to record its SERS and/or its Rayleigh scattering images in the different phases of its full cycle¹ , or as it dies if given cancer drugs², and enabled us to follow the dynamics of drug delivery³ and measure the relative efficacy of different cancer drugs^4,5using either Rayleigh or SERS method of detection. Finally, SERS technique was used in developing a technique that enabled us to follow the time profile of the different processes involved in the death mechanism⁶ of a cancer cell caused by use of a cancer drug.

Gold colloids are a class of interesting plasmonic particles in which the optical properties can be tailored by the size and shape of the nanostructure. In particular, gold nanoparticles act as optical antennas, increasing the local electric fields greater than an order of magnitude. This phenomenon is of great use in creating ultrasensitive biological probes utilizing surface enhanced Raman spectroscopy (SERS), where signal intensities scale roughly to the fourth power of the local field. The sensitivity of gold SERS probes can reach femtomolar levels which is more than 1000 times that of conventional medical imaging technologies such as magnetic resonance imaging (MRI), positron emission tomography (PET), or x-ray computed tomography (CT). In addition, gold exhibits low biological toxicity due to the relatively inert chemical nature of the material. In this talk the rational design of gold SERS probes for biological applications and successes in bio-analytical and in vivo imaging applications will be discussed along with obstacles which need to be overcome such as strategies to increase the specificity of targeted probes and designing structures which can be eliminated/excreted from the body after intravenous administration.

Gold nanoparticles with controlled size and shape have been successfully used as building blocks for the fabrications of 2- and 3-D nanoparticle arrays (or mesophases) for the development of ‘artificial solids&’ with collective optical and electronic properties. Nanoparticle surface chemistry plays a key role in control of microstructures of the nanoparticle arrays such as mesophase, interparticle separation, etc. To date, major efforts have mostly focused on the synthesis of nanoparticles that are stabilized with alkane ligands (CH₃(CH₂)_nR, R=SH, NH₂, PO, etc). Self-assembly of these nanoparticles and formation of ordered nanocrystal arrays are driven by hydrophobic (or van der Waal) interactions of interdigitated alkane chains between nanoparticles. In this presentation, I will present a self-assembled method that was developed at Sandia National Labs to synthesize gold nanoparticle-micelles and to assemble them into ordered, three-dimensional, nanoparticle arrays. Gold nanoparticle-micelles were synthesized through encapsulation of organic monolayer (alkane chains) derivatized, hydrophobic nanoparticles within the hydrophobic interior of surfactant/lipid micelles. This approach is simple, general, and can be used to synthesize water-soluble metal, semiconductor, and magnetic nanoparticles with different shapes. The flexible nanoparticle-micelle surface chemistry provides unique opportunity for subsequent self-assembly through hydrogen-bonding, charge interactions, etc. and formation of highly ordered nanocrystal arrays with controlled mesophase, orientation, and material forms (film, powder, particle). These robust, 3-D nanoparticle mesophase solids are demonstrated with great potentials for development of collective optical and electronic phenomena, and, importantly, for the integration of nanocrystal arrays into device architectures.
Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy&’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Advances in morphological control during nanoparticle synthesis have given rise to increasingly sophisticated structures. Recently, the integration of these nanoparticles into biosensor designs has enabled high-sensitivity devices that are low cost and easy to fabricate. Specifically, paper-based devices for the detection of biomarkers that leverage changes in the localized-surface plasmon resonance (LSPR) of nanoparticles have been demonstrated in proof-of-concept systems. However, the ability to create a broadly applicable detection platform requires generalized surface chemistries (e.g. anti-bodies, DNA, aptamers, peptides) that can easily modified to be made specific to the analyte of interest. Here, we describe developments on the synthesis and application of paper-based biosensors consisting of anisotropic nanoparticles modified with biological recognition elements (BREs) for selective and sensitive biodetection. The molecular specificity of BREs combined with the LSPR sensitivity of gold nanoparticles is used in the selective detection of a human performance biomarker Neuropeptide Y in biological fluids on a paper-based device. The simplicity in fabrication and versatility in function makes this a highly promising platform for point-of-care diagnostics.

Patchy particles exhibit multiple surface functionalities leading to novel chemical and physical properties which define them as exciting anisotropic building blocks for the self- and directed-assembly of a broad range of functional materials. Currently, the lack of scalable synthetic procedures for producing patchy particles represents a major obstacle to exploitation. Recently we made the discovery that ultrathin metal patchy coatings can be readily produced on nonmetallic spherical core nanoparticles. Our approach is based on the electroless reduction of metal ions under conditions which promote heterogeneous nucleation onto the cores. A limitation of this approach is the stochastic nature of nucleation which means that the location of nuclei formation cannot be addressed by the process variables when spherical core particles are used. However, control of the position and number of patches is crucial in order to obtain patchy particles that can be assembled into predictable mesostructures. In order to dictate the location of patch formation while maintaining the simplicity of the process, we propose in this contribution the use of shape anisotropic core particles. Classical thermodynamics teaches us that the local curvature of a particle surface affects both its chemical potential and ability to promote heterogeneous growth. More specifically it would be expected that heterogeneous nucleation will occur preferentially on parts of a particle surface which lead to a higher net reduction in free energy. To test this idea we chose triangular gold nanoprisms as core particle on account of their combination of both low and high curvature or their faces and edges respectively. The approximately 6 nm thick and 150 nm edge length gold nanoprisms were produced using a seeded growth method and were subsequently coated in a two-step process with a layer of amorphous silica. The latter coating was intended to mask the chemical properties of the gold core and to provide a surface analogous to the silica spheres used in our earlier work. To explore the influence of the silica coated gold nanoprisms on patch formation, we carried out a modified version of the Tollen&’s reaction, systematically varying several process parameters including temperature, reducing agent and metal precursor concentration and rate of addition. We also investigated the influence of the silica thicknesses on the gold core. Our results indicated unequivocally that nucleation occurs preferentially at the prism edges. Furthermore, it was found that the separation of nucleation from growth was essential in order to control patch monodispersity. The latter was achieved via dropwise addition of the reducing agent. Under these conditions very well-defined triangular ring-like silver patches were formed on the silica coated gold nanoprisms. Besides electron microscopy we confirm the obtained structure using optical spectroscopy compared with finite element method electrodynamic simulations.

The invention of the proton exchange membrane fuel cell (PEMFC) occurred over a half-century ago and, in spite of extensive developments of its science and its components, its widespread commercialization remains a challenge. Currently, improvements in the durability, performance and manufacturing cost of PEMFCs are required - all of which can be addressed with advances in catalyst function. Platinum (Pt) for example has long acted as the benchmark anodic and cathodic fuel cell catalyst, which makes resulting fuel cell devices less efficient due to Pt poisoning and slow kinetics for the oxygen reduction reaction (ORR). Recent work in our laboratories has shown that a block copolymer template-directed synthesis of mono- and bimetallic nanoparticle catalysts is an attractive method for preparing high performance FCscatalysts. This presentation will describe our investigation of the underlying principles for prescribing the size, spacing and composition of nanocatalysts that can be isolated from self-assembled block copolymers loaded with metallic anions and/or cations.
The approach offers several advantageous synthetic features compared to solvothermal catalyst synthesis. The approach is highly flexible and allows for the preparation and screening of a variety of mono- and bimetallic catalysts particles. The method allows for the fine-tuning of the catalyst activity by selection of the block copolymer template precursor. The equivalent exercise of isolating and refining catalysts by solvothermal or direct metallurgy techniques often requires a vigilant redesign of the experimental conditions (ligands, solvents, temperature, pressure, new metal salt precursors, etc.) that control the growth and reduction kinetics of the reagents that create the nanocatalysts. Accordingly, the electrocatalytic activity of block copolymer template-directed PtAu bimetallic catalysts is presented. Details for the methanol oxidation reactions and oxygen reduction reaction and tolerance to catalyst poisoning are discussed. Preliminary details on a structure-activity relationship between nanocatalysts and their self-assembled block copolymer templates is also discussed.

We model the mechanical response of alkanethiol-passivated gold nanoparticle superlattice (supercrystal) at ambient and elevated pressures using large-scale molecular dynamics simulation. Because of the important roles of soft organic ligands in mechanical response, the supercrystals exhibit entropic viscoelasticity during compression at ambient pressure. Applying a hydrostatic pressure of several hundred megapascals on the superlattice, combined with a critical deviatoric stress of the same order along the [110] direction of the face-centered-cubic supercrystal, can drive the room-temperature sintering ("fusion") of gold nanoparticles into ordered gold nanowire arrays. We discuss the molecular-level mechanism of such phenomena and map out a nonequilibrium stress-driven processing diagram, which reveals a region in stress space where fusion of nanoparticles can occur, instead of other competing plasticity or phase transformation processes in the supercrystal. We further demonstrate that, for silver-gold (Ag-Au) binary nanoparticle superlattices in sodium chloride-type superstructure, stress-driven fusion along the [100] direction leads to the ordered formation of Ag-Au multijunction nanowire arrays.

Synthesis of diverse gold nanostructures, such as nanospheres, nanorods, nanowires, and controlling both their nanoscale and macroscale organization are fundamental objectives in gold nanotechnology. The predominant strategy for construction of gold nanostructures in solution (also referred to as “bottom-up” approach) involves incubation of Au ions with reducing agents, with further addition of stabilizing agents for preventing aggregation and/or modulating the structural features of the nanoparticles assembled. This generic approach is limited when control over nanoparticle dimensionalities is essential, or when macroscopically-organized nano-structured assemblies are desired. We present a new “bottom-up” synthetic approach for producing Au nanostructures and for achieving long-range Au organization - using gold thiocyanate [Au(SCN)₄^1-] as a molecular precursor. We demonstrate that Au(SCN)₄^1- spontaneously forms diverse nanostructures, including extremely long, ordered nanowires, surface-supported nano-ribbons, and “nano-worms”. Furthermore, the Au(SCN)₄^1- complex enables construction of thin nano-structured films and macroscopically patterned structures via single step reaction pathways in the presence of amine-modified surfaces. Importantly, unlike virtually all solution-based Au nanoparticle synthesis schemes, in the Au thiocyanate system the ligand is the source of reducing electrons, rather than externally-added reducing agents. In a broader context, this work introduces a new route for fabricating gold nanostructures in which ligand-enabled crystallization/reduction could open the way to a variety of reaction pathways and applications.
Selected references:
1. “Patterned transparent conductive Au films through direct reduction of gold thiocyanate”
Ahiud Morag, Natalya Froumin, Dimitry Mogiliansky, Vladimir Ezersky, Edith Beilis, Shachar Richter, Raz Jelinek, Advanced Functional Materials, 2013, 23, 5663-5668.
2. “Transparent, conductive gold nanowire network assembled from soluble Au thiocyanate”, Ahiud Morag, Vladimir Ezersky, Natalya Froumin, Dimitry Mogiliansky, Raz Jelinek, ChemComm, 2013, 49(76), 8552-8554.
3. “Functional Gold Nanofibers Assembled on a Peptide Template through Single-step Reduction of Au(III) Thiocyanate”, T. P. Vinod, Shlomo Zarzhitsky, Ahiud Morag, Leila Zeiri, Yael Levi-Kalisman, Hanna Rapaport, Raz Jelinek, Nanoscale, 2013, 5, 10487-10493.

Metal nanoclusters containing tens to hundreds of metal atoms lie in an important regime that bridges molecular compounds and solid-state materials. It is found that during the growth process, clusters with certain “magic number” of metal atoms are extremely stable and more abundant than others. Understanding the formation of magic numbers and their growth pattern are two major focuses in metal nanocluster research. Recently, we found an intriguing “magic number series” in the thiolate-protected gold nanoclusters (denoted as Au_n(SR)_m), including the Au₂₀(SR)₁₆, Au₂₈(SR)₂₀, Au₃₆(SR)₂₄ and Au₄₄(SR)₂₈ with a uniform spacing of Au₈(SR)₄ between the adjacent sizes. Based on their crystallographic structures, we further found that the clusters in the magic series share the similar structure-construction rules, and the neat progression of magic numbers is a reflection of the uniform growth process of the clusters. Our results provide the first structure-evolution pattern of gold nanoclusters.

The self-assembly of metal nanoparticles driven by interactions of surface-anchored ligands such as DNA, multivalent thiolates, and proteins results in 1-, 2-, and 3-dimensional nanoparticle superstructures. In the case of widely studied gold nanoparticle superstructures, the assembly is almost always mediated by thiol-anchored ligands. This results in largely static and stable structures due to the comparatively strong Au-S bond precluding dynamic events. We report generalized assembly of gold nanoclusters by the non-thiolate ligand diglyme into discrete and dynamic structures. A modified Brust-Schiffrin synthesis alters the surface of the resulting thiolate-protected gold nanoclusters by incorporating a single molecule of diglyme into the shell layer. To understand this surprising phenomenon, the assembly of Au₂₀(SC₂H₄Ph)₁₅-diglyme into Au₂₀(SC₂H₄Ph)₁₅-diglyme-Au₂₀(SC₂H₄Ph)₁₅ is explored in detail. The dimerization is validated by size exclusion chromatography, mass spectrometry, IR spectroscopy, calorimetry, and electron microscopy. Assembly is enabled through weak oxygen-gold interactions between the terminal methoxy groups in diglyme and the cluster cores. We establish a dissociation constant for dimer to monomer conversion of 20.4 µM. Theoretical models validated by transient absorption spectroscopy predict a low-spin monomer and a high-spin dimer. Close spatial coupling allows electron delocalization between the nanoparticle cores, resulting in emergent optical, electronic, and structural properties. This new generic route for dynamic assemblies of gold nanoparticles has applications in sensing, plasmonics and biology that are absent in discrete metal nanoparticles.

The advances of metallic nanowire including 1D confinement of electron transport properties and high surface area enable its application in optical waveguide, lasers, sensors and nanoelectronics. Particularly, ultrathin gold nanowire (ultAuNW) offers a large variety of potential applications thanks to its unique optical properties,conductivity, chemical activity and discrete plasticity. Notably, even though enormous effort has been applied to generate AuNWs basing on the classic cetyltrimethylammonium bromide(CTAB) templated strategy,